A model is the single, definitive source of data about your data. It contains the essential fields and behaviors of the data you’re storing. Generally, each model maps to a single database table.
The basics:
A companion to this document is the official repository of model examples. (In the Django source distribution, these examples are in the tests/modeltests directory.)
This example model defines a Person, which has a first_name and last_name:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
first_name and last_name are fields of the model. Each field is specified as a class attribute, and each attribute maps to a database column.
The above Person model would create a database table like this:
CREATE TABLE myapp_person (
"id" serial NOT NULL PRIMARY KEY,
"first_name" varchar(30) NOT NULL,
"last_name" varchar(30) NOT NULL
);
Some technical notes:
The most important part of a model — and the only required part of a model — is the list of database fields it defines. Fields are specified by class attributes.
Example:
class Musician(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
instrument = models.CharField(max_length=100)
class Album(models.Model):
artist = models.ForeignKey(Musician)
name = models.CharField(max_length=100)
release_date = models.DateField()
num_stars = models.IntegerField()
Django places only two restrictions on model field names:
A field name cannot be a Python reserved word, because that would result in a Python syntax error. For example:
class Example(models.Model):
pass = models.IntegerField() # 'pass' is a reserved word!
A field name cannot contain more than one underscore in a row, due to the way Django’s query lookup syntax works. For example:
class Example(models.Model):
foo__bar = models.IntegerField() # 'foo__bar' has two underscores!
These limitations can be worked around, though, because your field name doesn’t necessarily have to match your database column name. See db_column below.
SQL reserved words, such as join, where or select, are allowed as model field names, because Django escapes all database table names and column names in every underlying SQL query. It uses the quoting syntax of your particular database engine.
Each field in your model should be an instance of the appropriate Field class. Django uses the field class types to determine a few things:
Here are all available field types:
An IntegerField that automatically increments according to available IDs. You usually won’t need to use this directly; a primary key field will automatically be added to your model if you don’t specify otherwise. See Automatic primary key fields.
A true/false field.
The admin represents this as a checkbox.
A string field, for small- to large-sized strings.
For large amounts of text, use TextField.
The admin represents this as an <input type="text"> (a single-line input).
CharField has an extra required argument, max_length, the maximum length (in characters) of the field. The max_length is enforced at the database level and in Django’s validation.
Django veterans: Note that the argument is now called max_length to provide consistency throughout Django. There is full legacy support for the old maxlength argument, but max_length is preferred.
A field of integers separated by commas. As in CharField, the max_length argument is required.
A date field. Has a few extra optional arguments:
| Argument | Description |
|---|---|
| auto_now | Automatically set the field to now every time the object is saved. Useful for “last-modified” timestamps. Note that the current date is always used; it’s not just a default value that you can override. |
| auto_now_add | Automatically set the field to now when the object is first created. Useful for creation of timestamps. Note that the current date is always used; it’s not just a default value that you can override. |
The admin represents this as an <input type="text"> with a JavaScript calendar, and a shortcut for “Today.” The JavaScript calendar will always start the week on a Sunday.
A date and time field. Takes the same extra options as DateField.
The admin represents this as two <input type="text"> fields, with JavaScript shortcuts.
New in Django development version
A fixed-precision decimal number, represented in Python by a Decimal instance. Has two required arguments:
| Argument | Description |
|---|---|
| max_digits | The maximum number of digits allowed in the number. |
| decimal_places | The number of decimal places to store with the number. |
For example, to store numbers up to 999 with a resolution of 2 decimal places, you’d use:
models.DecimalField(..., max_digits=5, decimal_places=2)
And to store numbers up to approximately one billion with a resolution of 10 decimal places:
models.DecimalField(..., max_digits=19, decimal_places=10)
The admin represents this as an <input type="text"> (a single-line input).
A CharField that checks that the value is a valid e-mail address.
In Django 0.96, this doesn’t accept max_length; its max_length is automatically set to 75. In the Django development version, max_length is set to 75 by default, but you can specify it to override default behavior.
A file-upload field. Has one required argument:
| Argument | Description |
|---|---|
| upload_to | A local filesystem path that will be appended to your MEDIA_ROOT setting to determine the output of the get_<fieldname>_url() helper function. |
This path may contain strftime formatting, which will be replaced by the date/time of the file upload (so that uploaded files don’t fill up the given directory).
The admin represents this field as an <input type="file"> (a file-upload widget).
Using a FileField or an ImageField (see below) in a model takes a few steps:
For example, say your MEDIA_ROOT is set to '/home/media', and upload_to is set to 'photos/%Y/%m/%d'. The '%Y/%m/%d' part of upload_to is strftime formatting; '%Y' is the four-digit year, '%m' is the two-digit month and '%d' is the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in the directory /home/media/photos/2007/01/15.
If you want to retrieve the upload file’s on-disk filename, or a URL that refers to that file, or the file’s size, you can use the get_FOO_filename(), get_FOO_url() and get_FOO_size() methods. They are all documented here.
Note that whenever you deal with uploaded files, you should pay close attention to where you’re uploading them and what type of files they are, to avoid security holes. Validate all uploaded files so that you’re sure the files are what you think they are. For example, if you blindly let somebody upload files, without validation, to a directory that’s within your Web server’s document root, then somebody could upload a CGI or PHP script and execute that script by visiting its URL on your site. Don’t allow that.
New in development version: By default, FileField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.
A field whose choices are limited to the filenames in a certain directory on the filesystem. Has three special arguments, of which the first is required:
| Argument | Description |
|---|---|
| path | Required. The absolute filesystem path to a directory from which this FilePathField should get its choices. Example: "/home/images". |
| match | Optional. A regular expression, as a string, that FilePathField will use to filter filenames. Note that the regex will be applied to the base filename, not the full path. Example: "foo.*\.txt$", which will match a file called foo23.txt but not bar.txt or foo23.gif. |
| recursive | Optional. Either True or False. Default is False. Specifies whether all subdirectories of path should be included. |
Of course, these arguments can be used together.
The one potential gotcha is that match applies to the base filename, not the full path. So, this example:
FilePathField(path="/home/images", match="foo.*", recursive=True)
…will match /home/images/foo.gif but not /home/images/foo/bar.gif because the match applies to the base filename (foo.gif and bar.gif).
New in development version: By default, FilePathField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.
Changed in Django development version
A floating-point number represented in Python by a float instance.
The admin represents this as an <input type="text"> (a single-line input).
NOTE: The semantics of FloatField have changed in the Django development version. See the Django 0.96 documentation for the old behavior.
Like FileField, but validates that the uploaded object is a valid image. Has two extra optional arguments, height_field and width_field, which, if set, will be auto-populated with the height and width of the image each time a model instance is saved.
In addition to the special get_FOO_* methods that are available for FileField, an ImageField also has get_FOO_height() and get_FOO_width() methods. These are documented elsewhere.
Requires the Python Imaging Library.
New in development version: By default, ImageField instances are created as varchar(100) columns in your database. As with other fields, you can change the maximum length using the max_length argument.
An integer.
The admin represents this as an <input type="text"> (a single-line input).
An IP address, in string format (e.g. “192.0.2.30”).
The admin represents this as an <input type="text"> (a single-line input).
Like a BooleanField, but allows NULL as one of the options. Use this instead of a BooleanField with null=True.
The admin represents this as a <select> box with “Unknown”, “Yes” and “No” choices.
A CharField that checks that the value is a valid U.S.A.-style phone number (in the format XXX-XXX-XXXX).
Like an IntegerField, but must be positive.
Like a PositiveIntegerField, but only allows values under a certain (database-dependent) point.
“Slug” is a newspaper term. A slug is a short label for something, containing only letters, numbers, underscores or hyphens. They’re generally used in URLs.
Like a CharField, you can specify max_length. If max_length is not specified, Django will use a default length of 50.
Implies db_index=True.
Like an IntegerField, but only allows values under a certain (database-dependent) point.
A large text field.
The admin represents this as a <textarea> (a multi-line input).
A time. Accepts the same auto-population options as DateField and DateTimeField.
The admin represents this as an <input type="text"> with some JavaScript shortcuts.
A field for a URL. If the verify_exists option is True (default), the URL given will be checked for existence (i.e., the URL actually loads and doesn’t give a 404 response).
The admin represents this as an <input type="text"> (a single-line input).
URLField takes an optional argument, max_length, the maximum length (in characters) of the field. The maximum length is enforced at the database level and in Django’s validation. If you don’t specify max_length, a default of 200 is used.
A two-letter U.S. state abbreviation.
The admin represents this as an <input type="text"> (a single-line input).
A TextField that checks that the value is valid XML that matches a given schema. Takes one required argument, schema_path, which is the filesystem path to a RelaxNG schema against which to validate the field.
The following arguments are available to all field types. All are optional.
If True, Django will store empty values as NULL in the database. Default is False.
Note that empty string values will always get stored as empty strings, not as NULL. Only use null=True for non-string fields such as integers, booleans and dates. For both types of fields, you will also need to set blank=True if you wish to permit empty values in forms, as the null parameter only affects database storage (see blank, below).
Avoid using null on string-based fields such as CharField and TextField unless you have an excellent reason. If a string-based field has null=True, that means it has two possible values for “no data”: NULL, and the empty string. In most cases, it’s redundant to have two possible values for “no data;” Django convention is to use the empty string, not NULL.
Note
When using the Oracle database backend, the null=True option will be coerced for string-based fields that can blank, and the value NULL will be stored to denote the empty string.
If True, the field is allowed to be blank. Default is False.
Note that this is different than null. null is purely database-related, whereas blank is validation-related. If a field has blank=True, validation on Django’s admin site will allow entry of an empty value. If a field has blank=False, the field will be required.
An iterable (e.g., a list or tuple) of 2-tuples to use as choices for this field.
If this is given, Django’s admin will use a select box instead of the standard text field and will limit choices to the choices given.
A choices list looks like this:
YEAR_IN_SCHOOL_CHOICES = (
('FR', 'Freshman'),
('SO', 'Sophomore'),
('JR', 'Junior'),
('SR', 'Senior'),
('GR', 'Graduate'),
)
The first element in each tuple is the actual value to be stored. The second element is the human-readable name for the option.
The choices list can be defined either as part of your model class:
class Foo(models.Model):
GENDER_CHOICES = (
('M', 'Male'),
('F', 'Female'),
)
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
or outside your model class altogether:
GENDER_CHOICES = (
('M', 'Male'),
('F', 'Female'),
)
class Foo(models.Model):
gender = models.CharField(max_length=1, choices=GENDER_CHOICES)
You can also collect your available choices into named groups that can be used for organizational purposes:
MEDIA_CHOICES = (
('Audio', (
('vinyl', 'Vinyl'),
('cd', 'CD'),
)
),
('Video', (
('vhs', 'VHS Tape'),
('dvd', 'DVD'),
)
),
('unknown', 'Unknown'),
)
The first element in each tuple is the name to apply to the group. The second element is an iterable of 2-tuples, with each 2-tuple containing a value and a human-readable name for an option. Grouped options may be combined with ungrouped options within a single list (such as the unknown option in this example).
For each model field that has choices set, Django will add a method to retrieve the human-readable name for the field’s current value. See get_FOO_display in the database API documentation.
Finally, note that choices can be any iterable object — not necessarily a list or tuple. This lets you construct choices dynamically. But if you find yourself hacking choices to be dynamic, you’re probably better off using a proper database table with a ForeignKey. choices is meant for static data that doesn’t change much, if ever.
For objects that are edited inline to a related object.
In the Django admin, if all “core” fields in an inline-edited object are cleared, the object will be deleted.
It is an error to have an inline-editable relation without at least one core=True field.
Please note that each field marked “core” is treated as a required field by the Django admin site. Essentially, this means you should put core=True on all required fields in your related object that is being edited inline.
The name of the database column to use for this field. If this isn’t given, Django will use the field’s name.
If your database column name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names — notably, the hyphen — that’s OK. Django quotes column and table names behind the scenes.
If True, django-admin.py sqlindexes will output a CREATE INDEX statement for this field.
New in Django development version
The name of the database tablespace to use for this field’s index, if this field is indexed. The default is the project’s DEFAULT_INDEX_TABLESPACE setting, if set, or the db_tablespace of the model, if any. If the backend doesn’t support tablespaces, this option is ignored.
The default value for the field. This can be a value or a callable object. If callable it will be called every time a new object is created.
If False, the field will not be editable in the admin or via form processing using the object’s AddManipulator or ChangeManipulator classes. Default is True.
Extra “help” text to be displayed under the field on the object’s admin form. It’s useful for documentation even if your object doesn’t have an admin form.
Note that this value is not HTML-escaped when it’s displayed in the admin interface. This lets you include HTML in help_text if you so desire. For example:
help_text="Please use the following format: <em>YYYY-MM-DD</em>."
If True, this field is the primary key for the model.
If you don’t specify primary_key=True for any fields in your model, Django will automatically add this field:
id = models.AutoField('ID', primary_key=True)
Thus, you don’t need to set primary_key=True on any of your fields unless you want to override the default primary-key behavior.
primary_key=True implies null=False and unique=True. Only one primary key is allowed on an object.
If True, this field must be unique throughout the table.
This is enforced at the database level and at the Django admin-form level. If you try to save a model with a duplicate value in a unique field, a django.db.IntegrityError will be raised by the model’s save() method.
Set this to the name of a DateField or DateTimeField to require that this field be unique for the value of the date field.
For example, if you have a field title that has unique_for_date="pub_date", then Django wouldn’t allow the entry of two records with the same title and pub_date.
This is enforced at the Django admin-form level but not at the database level.
Like unique_for_date, but requires the field to be unique with respect to the month.
Like unique_for_date and unique_for_month.
A list of extra validators to apply to the field. Each should be a callable that takes the parameters field_data, all_data and raises django.core.validators.ValidationError for errors. (See the validator docs.)
Django comes with quite a few validators. They’re in django.core.validators.
Each field type, except for ForeignKey, ManyToManyField and OneToOneField, takes an optional first positional argument — a verbose name. If the verbose name isn’t given, Django will automatically create it using the field’s attribute name, converting underscores to spaces.
In this example, the verbose name is "Person's first name":
first_name = models.CharField("Person's first name", max_length=30)
In this example, the verbose name is "first name":
first_name = models.CharField(max_length=30)
ForeignKey, ManyToManyField and OneToOneField require the first argument to be a model class, so use the verbose_name keyword argument:
poll = models.ForeignKey(Poll, verbose_name="the related poll") sites = models.ManyToManyField(Site, verbose_name="list of sites") place = models.OneToOneField(Place, verbose_name="related place")
Convention is not to capitalize the first letter of the verbose_name. Django will automatically capitalize the first letter where it needs to.
Clearly, the power of relational databases lies in relating tables to each other. Django offers ways to define the three most common types of database relationships: Many-to-one, many-to-many and one-to-one.
To define a many-to-one relationship, use ForeignKey. You use it just like any other Field type: by including it as a class attribute of your model.
ForeignKey requires a positional argument: the class to which the model is related.
For example, if a Car model has a Manufacturer — that is, a Manufacturer makes multiple cars but each Car only has one Manufacturer — use the following definitions:
class Manufacturer(models.Model):
# ...
class Car(models.Model):
manufacturer = models.ForeignKey(Manufacturer)
# ...
To create a recursive relationship — an object that has a many-to-one relationship with itself — use models.ForeignKey('self').
If you need to create a relationship on a model that has not yet been defined, you can use the name of the model, rather than the model object itself:
class Car(models.Model):
manufacturer = models.ForeignKey('Manufacturer')
# ...
class Manufacturer(models.Model):
# ...
Note, however, that this only refers to models in the same models.py file — you cannot use a string to reference a model defined in another application or imported from elsewhere.
New in Django development version: To refer to models defined in another application, you must instead explicitly specify the application label. For example, if the Manufacturer model above is defined in another application called production, you’d need to use:
class Car(models.Model):
manufacturer = models.ForeignKey('production.Manufacturer')
Behind the scenes, Django appends "_id" to the field name to create its database column name. In the above example, the database table for the Car model will have a manufacturer_id column. (You can change this explicitly by specifying db_column; see db_column below.) However, your code should never have to deal with the database column name, unless you write custom SQL. You’ll always deal with the field names of your model object.
It’s suggested, but not required, that the name of a ForeignKey field (manufacturer in the example above) be the name of the model, lowercase. You can, of course, call the field whatever you want. For example:
class Car(models.Model):
company_that_makes_it = models.ForeignKey(Manufacturer)
# ...
See the Many-to-one relationship model example for a full example.
ForeignKey fields take a number of extra arguments for defining how the relationship should work. All are optional:
| Argument | Description |
|---|---|
| limit_choices_to | A dictionary of lookup arguments and values (see the Database API reference) that limit the available admin choices for this object. Use this with functions from the Python datetime module to limit choices of objects by date. For example:
limit_choices_to = {'pub_date__lte': datetime.now}
only allows the choice of related objects with a pub_date before the current date/time to be chosen. Instead of a dictionary this can also be a Q object (an object with a get_sql() method) for more complex queries. Not compatible with edit_inline. |
| related_name | The name to use for the relation from the related object back to this one. See the related objects documentation for a full explanation and example. If using this in an abstract base class, be sure to read the extra notes in that section about related_name. |
| to_field | The field on the related object that the relation is to. By default, Django uses the primary key of the related object. |
To define a many-to-many relationship, use ManyToManyField. You use it just like any other Field type: by including it as a class attribute of your model.
ManyToManyField requires a positional argument: the class to which the model is related.
For example, if a Pizza has multiple Topping objects — that is, a Topping can be on multiple pizzas and each Pizza has multiple toppings — here’s how you’d represent that:
class Topping(models.Model):
# ...
class Pizza(models.Model):
# ...
toppings = models.ManyToManyField(Topping)
As with ForeignKey, a relationship to self can be defined by using the string 'self' instead of the model name, and you can refer to as-yet undefined models by using a string containing the model name. However, you can only use strings to refer to models in the same models.py file — you cannot use a string to reference a model in a different application, or to reference a model that has been imported from elsewhere.
It’s suggested, but not required, that the name of a ManyToManyField (toppings in the example above) be a plural describing the set of related model objects.
Behind the scenes, Django creates an intermediary join table to represent the many-to-many relationship.
It doesn’t matter which model gets the ManyToManyField, but you only need it in one of the models — not in both.
Generally, ManyToManyField instances should go in the object that’s going to be edited in the admin interface, if you’re using Django’s admin. In the above example, toppings is in Pizza (rather than Topping having a pizzas ManyToManyField ) because it’s more natural to think about a Pizza having toppings than a topping being on multiple pizzas. The way it’s set up above, the Pizza admin form would let users select the toppings.
See the Many-to-many relationship model example for a full example.
ManyToManyField objects take a number of extra arguments for defining how the relationship should work. All are optional:
| Argument | Description |
|---|---|
| related_name | See the description under ForeignKey above. |
| limit_choices_to | See the description under ForeignKey above. |
| symmetrical | Only used in the definition of ManyToManyFields on self. Consider the following model:
class Person(models.Model):
friends = models.ManyToManyField("self")
When Django processes this model, it identifies that it has a ManyToManyField on itself, and as a result, it doesn’t add a person_set attribute to the Person class. Instead, the ManyToManyField is assumed to be symmetrical — that is, if I am your friend, then you are my friend. If you do not want symmetry in ManyToMany relationships with self, set symmetrical to False. This will force Django to add the descriptor for the reverse relationship, allowing ManyToMany relationships to be non-symmetrical. |
| db_table | The name of the table to create for storing the many-to-many data. If this is not provided, Django will assume a default name based upon the names of the two tables being joined. |
To define a one-to-one relationship, use OneToOneField. You use it just like any other Field type: by including it as a class attribute of your model.
This is most useful on the primary key of an object when that object “extends” another object in some way.
OneToOneField requires a positional argument: the class to which the model is related.
For example, if you’re building a database of “places”, you would build pretty standard stuff such as address, phone number, etc. in the database. Then, if you wanted to build a database of restaurants on top of the places, instead of repeating yourself and replicating those fields in the Restaurant model, you could make Restaurant have a OneToOneField to Place (because a restaurant “is-a” place).
As with ForeignKey, a relationship to self can be defined by using the string "self" instead of the model name; references to as-yet undefined models can be made by using a string containing the model name.
Finally, OneToOneField takes the following extra option:
| Argument | Description |
|---|---|
| parent_link | When True and used in a model inherited from another model, indicates that this field should be used as the link from the child back to the parent. See Model inheritance for more details. New in Django development version |
New in Django development version: OneToOneField classes used to automatically become the primary key on a model. This is no longer true, although you can manually pass in the primary_key attribute if you like. Thus, it’s now possible to have multiple fields of type OneToOneField on a single model.
See the One-to-one relationship model example for a full example.
New in Django development version
If one of the existing model fields cannot be used to fit your purposes, or if you wish to take advantage of some less common database column types, you can create your own field class. Full coverage of creating your own fields is provided in the Custom Model Fields documentation.
Give your model metadata by using an inner class Meta, like so:
class Foo(models.Model):
bar = models.CharField(max_length=30)
class Meta:
# ...
Model metadata is “anything that’s not a field”, such as ordering options, etc.
Here’s a list of all possible Meta options. No options are required. Adding class Meta to a model is completely optional.
New in Django development version
When set to True, denotes this model as an abstract base class. See Abstract base classes for more details. Defaults to False.
The name of the database table to use for the model:
db_table = 'music_album'
If this isn’t given, Django will use app_label + '_' + model_class_name. See “Table names” below for more.
If your database table name is an SQL reserved word, or contains characters that aren’t allowed in Python variable names — notably, the hyphen — that’s OK. Django quotes column and table names behind the scenes.
New in Django development version
The name of the database tablespace to use for the model. If the backend doesn’t support tablespaces, this option is ignored.
The name of a DateField or DateTimeField in the model. This specifies the default field to use in your model Manager‘s latest() method.
Example:
get_latest_by = "order_date"
See the docs for latest() for more.
Marks this object as “orderable” with respect to the given field. This is almost always used with related objects to allow them to be ordered with respect to a parent object. For example, if an Answer relates to a Question object, and a question has more than one answer, and the order of answers matters, you’d do this:
class Answer(models.Model):
question = models.ForeignKey(Question)
# ...
class Meta:
order_with_respect_to = 'question'
The default ordering for the object, for use when obtaining lists of objects:
ordering = ['-order_date']
This is a tuple or list of strings. Each string is a field name with an optional “-” prefix, which indicates descending order. Fields without a leading “-” will be ordered ascending. Use the string “?” to order randomly.
For example, to order by a pub_date field ascending, use this:
ordering = ['pub_date']
To order by pub_date descending, use this:
ordering = ['-pub_date']
To order by pub_date descending, then by author ascending, use this:
ordering = ['-pub_date', 'author']
See Specifying ordering for more examples.
Note that, regardless of how many fields are in ordering, the admin site uses only the first field.
Extra permissions to enter into the permissions table when creating this object. Add, delete and change permissions are automatically created for each object that has admin set. This example specifies an extra permission, can_deliver_pizzas:
permissions = (("can_deliver_pizzas", "Can deliver pizzas"),)
This is a list or tuple of 2-tuples in the format (permission_code, human_readable_permission_name).
Sets of field names that, taken together, must be unique:
unique_together = (("driver", "restaurant"),)
This is a list of lists of fields that must be unique when considered together. It’s used in the Django admin and is enforced at the database level (i.e., the appropriate UNIQUE statements are included in the CREATE TABLE statement).
All the fields specified in unique_together must be part of the current model. If you are using model inheritance, you cannot refer to fields from any parent classes in unique_together.
New in Django development version
For convenience, unique_together can be a single list when dealing with a single set of fields:
unique_together = ("driver", "restaurant")
A human-readable name for the object, singular:
verbose_name = "pizza"
If this isn’t given, Django will use a munged version of the class name: CamelCase becomes camel case.
The plural name for the object:
verbose_name_plural = "stories"
If this isn’t given, Django will use verbose_name + "s".
To save you time, Django automatically derives the name of the database table from the name of your model class and the app that contains it. A model’s database table name is constructed by joining the model’s “app label” — the name you used in manage.py startapp — to the model’s class name, with an underscore between them.
For example, if you have an app bookstore (as created by manage.py startapp bookstore), a model defined as class Book will have a database table named bookstore_book.
To override the database table name, use the db_table parameter in class Meta.
By default, Django gives each model the following field:
id = models.AutoField(primary_key=True)
This is an auto-incrementing primary key.
If you’d like to specify a custom primary key, just specify primary_key=True on one of your fields. If Django sees you’ve explicitly set primary_key, it won’t add the automatic id column.
Each model requires exactly one field to have primary_key=True.
New in Django development version
Regardless of whether you define a primary key field yourself, or let Django supply one for you, each model will have a property called pk. It behaves like a normal attribute on the model, but is actually an alias for whichever attribute is the primary key field for the model. You can read and set this value, just as you would for any other attribute, and it will update the correct field in the model.
A Manager is the interface through which database query operations are provided to Django models. At least one Manager exists for every model in a Django application.
The way Manager classes work is documented in the Retrieving objects section of the database API docs, but this section specifically touches on model options that customize Manager behavior.
By default, Django adds a Manager with the name objects to every Django model class. However, if you want to use objects as a field name, or if you want to use a name other than objects for the Manager, you can rename it on a per-model basis. To rename the Manager for a given class, define a class attribute of type models.Manager() on that model. For example:
from django.db import models
class Person(models.Model):
#...
people = models.Manager()
Using this example model, Person.objects will generate an AttributeError exception, but Person.people.all() will provide a list of all Person objects.
You can use a custom Manager in a particular model by extending the base Manager class and instantiating your custom Manager in your model.
There are two reasons you might want to customize a Manager: to add extra Manager methods, and/or to modify the initial QuerySet the Manager returns.
Adding extra Manager methods is the preferred way to add “table-level” functionality to your models. (For “row-level” functionality — i.e., functions that act on a single instance of a model object — use Model methods, not custom Manager methods.)
A custom Manager method can return anything you want. It doesn’t have to return a QuerySet.
For example, this custom Manager offers a method with_counts(), which returns a list of all OpinionPoll objects, each with an extra num_responses attribute that is the result of an aggregate query:
class PollManager(models.Manager):
def with_counts(self):
from django.db import connection
cursor = connection.cursor()
cursor.execute("""
SELECT p.id, p.question, p.poll_date, COUNT(*)
FROM polls_opinionpoll p, polls_response r
WHERE p.id = r.poll_id
GROUP BY 1, 2, 3
ORDER BY 3 DESC""")
result_list = []
for row in cursor.fetchall():
p = self.model(id=row[0], question=row[1], poll_date=row[2])
p.num_responses = row[3]
result_list.append(p)
return result_list
class OpinionPoll(models.Model):
question = models.CharField(max_length=200)
poll_date = models.DateField()
objects = PollManager()
class Response(models.Model):
poll = models.ForeignKey(Poll)
person_name = models.CharField(max_length=50)
response = models.TextField()
With this example, you’d use OpinionPoll.objects.with_counts() to return that list of OpinionPoll objects with num_responses attributes.
Another thing to note about this example is that Manager methods can access self.model to get the model class to which they’re attached.
A Manager‘s base QuerySet returns all objects in the system. For example, using this model:
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.CharField(max_length=50)
…the statement Book.objects.all() will return all books in the database.
You can override a Manager‘s base QuerySet by overriding the Manager.get_query_set() method. get_query_set() should return a QuerySet with the properties you require.
For example, the following model has two Managers — one that returns all objects, and one that returns only the books by Roald Dahl:
# First, define the Manager subclass.
class DahlBookManager(models.Manager):
def get_query_set(self):
return super(DahlBookManager, self).get_query_set().filter(author='Roald Dahl')
# Then hook it into the Book model explicitly.
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.CharField(max_length=50)
objects = models.Manager() # The default manager.
dahl_objects = DahlBookManager() # The Dahl-specific manager.
With this sample model, Book.objects.all() will return all books in the database, but Book.dahl_objects.all() will only return the ones written by Roald Dahl.
Of course, because get_query_set() returns a QuerySet object, you can use filter(), exclude() and all the other QuerySet methods on it. So these statements are all legal:
Book.dahl_objects.all() Book.dahl_objects.filter(title='Matilda') Book.dahl_objects.count()
This example also pointed out another interesting technique: using multiple managers on the same model. You can attach as many Manager() instances to a model as you’d like. This is an easy way to define common “filters” for your models.
For example:
class MaleManager(models.Manager):
def get_query_set(self):
return super(MaleManager, self).get_query_set().filter(sex='M')
class FemaleManager(models.Manager):
def get_query_set(self):
return super(FemaleManager, self).get_query_set().filter(sex='F')
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
sex = models.CharField(max_length=1, choices=(('M', 'Male'), ('F', 'Female')))
people = models.Manager()
men = MaleManager()
women = FemaleManager()
This example allows you to request Person.men.all(), Person.women.all(), and Person.people.all(), yielding predictable results.
If you use custom Manager objects, take note that the first Manager Django encounters (in the order in which they’re defined in the model) has a special status. Django interprets this first Manager defined in a class as the “default” Manager, and several parts of Django (though not the admin application) will use that Manager exclusively for that model. As a result, it’s often a good idea to be careful in your choice of default manager, in order to avoid a situation where overriding of get_query_set() results in an inability to retrieve objects you’d like to work with.
Define custom methods on a model to add custom “row-level” functionality to your objects. Whereas Manager methods are intended to do “table-wide” things, model methods should act on a particular model instance.
This is a valuable technique for keeping business logic in one place — the model.
For example, this model has a few custom methods:
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
birth_date = models.DateField()
address = models.CharField(max_length=100)
city = models.CharField(max_length=50)
state = models.USStateField() # Yes, this is America-centric...
def baby_boomer_status(self):
"Returns the person's baby-boomer status."
import datetime
if datetime.date(1945, 8, 1) <= self.birth_date <= datetime.date(1964, 12, 31):
return "Baby boomer"
if self.birth_date < datetime.date(1945, 8, 1):
return "Pre-boomer"
return "Post-boomer"
def is_midwestern(self):
"Returns True if this person is from the Midwest."
return self.state in ('IL', 'WI', 'MI', 'IN', 'OH', 'IA', 'MO')
def _get_full_name(self):
"Returns the person's full name."
return '%s %s' % (self.first_name, self.last_name)
full_name = property(_get_full_name)
The last method in this example is a property. Read more about properties.
A few object methods have special meaning:
__str__() is a Python “magic method” that defines what should be returned if you call str() on the object. Django uses str(obj) (or the related function, unicode(obj) — see below) in a number of places, most notably as the value displayed to render an object in the Django admin site and as the value inserted into a template when it displays an object. Thus, you should always return a nice, human-readable string for the object’s __str__. Although this isn’t required, it’s strongly encouraged (see the description of __unicode__, below, before putting __str__ methods everywhere).
For example:
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
def __str__(self):
# Note use of django.utils.encoding.smart_str() here because
# first_name and last_name will be unicode strings.
return smart_str('%s %s' % (self.first_name, self.last_name))
The __unicode__() method is called whenever you call unicode() on an object. Since Django’s database backends will return Unicode strings in your model’s attributes, you would normally want to write a __unicode__() method for your model. The example in the previous section could be written more simply as:
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
def __unicode__(self):
return u'%s %s' % (self.first_name, self.last_name)
If you define a __unicode__() method on your model and not a __str__() method, Django will automatically provide you with a __str__() that calls __unicode__() and then converts the result correctly to a UTF-8 encoded string object. This is recommended development practice: define only __unicode__() and let Django take care of the conversion to string objects when required.
Define a get_absolute_url() method to tell Django how to calculate the URL for an object. For example:
def get_absolute_url(self):
return "/people/%i/" % self.id
Django uses this in its admin interface. If an object defines get_absolute_url(), the object-editing page will have a “View on site” link that will jump you directly to the object’s public view, according to get_absolute_url().
Also, a couple of other bits of Django, such as the syndication feed framework, use get_absolute_url() as a convenience to reward people who’ve defined the method.
It’s good practice to use get_absolute_url() in templates, instead of hard-coding your objects’ URLs. For example, this template code is bad:
<a href="/people/{{ object.id }}/">{{ object.name }}</a>
But this template code is good:
<a href="{{ object.get_absolute_url }}">{{ object.name }}</a>
Note
The string you return from get_absolute_url() must contain only ASCII characters (required by the URI spec, RFC 2396) that have been URL-encoded, if necessary. Code and templates using get_absolute_url() should be able to use the result directly without needing to do any further processing. You may wish to use the django.utils.encoding.iri_to_uri() function to help with this if you are using unicode strings a lot.
The problem with the way we wrote get_absolute_url() above is that it slightly violates the DRY principle: the URL for this object is defined both in the URLConf file and in the model.
You can further decouple your models from the URLconf using the permalink decorator. This decorator is passed the view function, a list of positional parameters and (optionally) a dictionary of named parameters. Django then works out the correct full URL path using the URLconf, substituting the parameters you have given into the URL. For example, if your URLconf contained a line such as:
(r'^people/(\d+)/$', 'people.views.details'),
…your model could have a get_absolute_url method that looked like this:
from django.db.models import permalink
def get_absolute_url(self):
return ('people.views.details', [str(self.id)])
get_absolute_url = permalink(get_absolute_url)
Similarly, if you had a URLconf entry that looked like:
(r'/archive/(?P<year>\d{4})/(?P<month>\d{1,2})/(?P<day>\d{1,2})/$', archive_view)
…you could reference this using permalink() as follows:
def get_absolute_url(self):
return ('archive_view', (), {
'year': self.created.year,
'month': self.created.month,
'day': self.created.day})
get_absolute_url = permalink(get_absolute_url)
Notice that we specify an empty sequence for the second parameter in this case, because we only want to pass keyword parameters, not positional ones.
In this way, you’re tying the model’s absolute URL to the view that is used to display it, without repeating the URL information anywhere. You can still use the get_absolute_url method in templates, as before.
In some cases, such as the use of generic views or the re-use of custom views for multiple models, specifying the view function may confuse the reverse URL matcher (because multiple patterns point to the same view).
For that problem, Django has named URL patterns. Using a named URL pattern, it’s possible to give a name to a pattern, and then reference the name rather than the view function. A named URL pattern is defined by replacing the pattern tuple by a call to the url function):
from django.conf.urls.defaults import *
url(r'^people/(\d+)/$',
'django.views.generic.list_detail.object_detail',
name='people_view'),
…and then using that name to perform the reverse URL resolution instead of the view name:
from django.db.models import permalink
def get_absolute_url(self):
return ('people_view', [str(self.id)])
get_absolute_url = permalink(get_absolute_url)
More details on named URL patterns are in the URL dispatch documentation.
Feel free to write custom SQL statements in custom model methods and module-level methods. The object django.db.connection represents the current database connection. To use it, call connection.cursor() to get a cursor object. Then, call cursor.execute(sql, [params]) to execute the SQL and cursor.fetchone() or cursor.fetchall() to return the resulting rows. Example:
def my_custom_sql(self):
from django.db import connection
cursor = connection.cursor()
cursor.execute("SELECT foo FROM bar WHERE baz = %s", [self.baz])
row = cursor.fetchone()
return row
connection and cursor mostly implement the standard Python DB-API (except when it comes to transaction handling). If you’re not familiar with the Python DB-API, note that the SQL statement in cursor.execute() uses placeholders, "%s", rather than adding parameters directly within the SQL. If you use this technique, the underlying database library will automatically add quotes and escaping to your parameter(s) as necessary. (Also note that Django expects the "%s" placeholder, not the "?" placeholder, which is used by the SQLite Python bindings. This is for the sake of consistency and sanity.)
A final note: If all you want to do is a custom WHERE clause, you can just use the where, tables and params arguments to the standard lookup API. See Other lookup options.
As explained in the database API docs, each model gets a few methods automatically — most notably, save() and delete(). You can override these methods to alter behavior.
A classic use-case for overriding the built-in methods is if you want something to happen whenever you save an object. For example:
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self):
do_something()
super(Blog, self).save() # Call the "real" save() method.
do_something_else()
You can also prevent saving:
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self):
if self.name == "Yoko Ono's blog":
return # Yoko shall never have her own blog!
else:
super(Blog, self).save() # Call the "real" save() method.
New in Django development version
Model inheritance in Django works almost identically to the way normal class inheritance works in Python. The only decision you have to make is whether you want the parent models to be models in their own right (with their own database tables), or if the parents are just holders of common information that will only be visible through the child models.
Often, you will just want to use the parent class to hold information that you don’t want to have to type out for each child model. This class isn’t going to ever be used in isolation, so abstract base classes are what you’re after. However, if you’re subclassing an existing model (perhaps something from another application entirely), or want each model to have its own database table, multi-table inheritance is the way to go.
Abstract base classes are useful when you want to put some common information into a number of other models. You write your base class and put abstract=True in the Meta class. This model will then not be used to create any database table. Instead, when it is used as a base class for other models, its fields will be added to those of the child class. It is an error to have fields in the abstract base class with the same name as those in the child (and Django will raise an exception).
An example:
class CommonInfo(models.Model):
name = models.CharField(max_length=100)
age = models.PositiveIntegerField()
class Meta:
abstract = True
class Student(CommonInfo):
home_group = models.CharField(max_length=5)
The Student model will have three fields: name, age and home_group. The CommonInfo model cannot be used as a normal Django model, since it is an abstract base class. It does not generate a database table or have a manager or anything like that.
For many uses, this type of model inheritance will be exactly what you want. It provides a way to factor out common information at the Python level, whilst still only creating one database table per child model at the database level.
When an abstract base class is created, Django makes any Meta inner class you declared on the base class available as an attribute. If a child class does not declare its own Meta class, it will inherit the parent’s Meta. If the child wants to extend the parent’s Meta class, it can subclass it. For example:
class CommonInfo(models.Model):
...
class Meta:
abstract = True
ordering = ['name']
class Student(CommonInfo):
...
class Meta(CommonInfo.Meta):
db_table = 'student_info'
Django does make one adjustment to the Meta class of an abstract base class: before installing the Meta attribute, it sets abstract=False. This means that children of abstract base classes don’t automatically become abstract classes themselves. Of course, you can make an abstract base class that inherits from another abstract base class. You just need to remember to explicitly set abstract=True each time.
Some attributes won’t make sense to include in the Meta class of an abstract base class. For example, including db_table would mean that all the child classes (the ones that don’t specify their own Meta) would use the same database table, which is almost certainly not what you want.
The second type of model inheritance supported by Django is when each model in the hierarchy is a model all by itself. Each model corresponds to its own database table and can be queried and created individually. The inheritance relationship introduces links between the child model and each of its parents (via an automatically created OneToOneField). For example:
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
class Restaurant(Place):
serves_hot_dogs = models.BooleanField()
serves_pizza = models.BooleanField()
All of the fields of Place will also be available in Restaurant, although the data will reside in a different database table. So these are both possible:
>>> Place.objects.filter(name="Bob's Cafe") >>> Restaurant.objects.filter(name="Bob's Cafe")
If you have a Place that is also a Restaurant, you can get from the Place object to the Restaurant object by using the lower-case version of the model name:
>>> p = Place.objects.filter(name="Bob's Cafe") # If Bob's Cafe is a Restaurant object, this will give the child class: >>> p.restaurant <Restaurant: ...>
However, if p in the above example was not a Restaurant (it had been created directly as a Place object or was the parent of some other class), referring to p.restaurant would give an error.
In the multi-table inheritance situation, it doesn’t make sense for a child class to inherit from its parent’s Meta class. All the Meta options have already been applied to the parent class and applying them again would normally only lead to contradictory behaviour (this is in contrast with the abstract base class case, where the base class doesn’t exist in its own right).
So a child model does not have access to its parent’s Meta class. However, there are a few limited cases where the child inherits behaviour from the parent: if the child does not specify an ordering attribute or a get_latest_by attribute, it will inherit these from its parent.
If the parent has an ordering and you don’t want the child to have any natural ordering, you can explicitly set it to be empty:
class ChildModel(ParentModel):
...
class Meta:
# Remove parent's ordering effect
ordering = []
Because multi-table inheritance uses an implicit OneToOneField to link the child and the parent, it’s possible to move from the parent down to the child, as in the above example. However, this uses up the name that is the default related_name value for ForeignKey and ManyToManyField relations. If you are putting those type of relations on a subclass of another model, you must specify the related_name attribute on each such field. If you forget, Django will raise an error when you run manage.py validate or try to syncdb.
For example, using the above Place class again, let’s create another subclass with a ManyToManyField:
class Supplier(Place):
# Must specify related_name on all relations.
customers = models.ManyToManyField(Restaurant,
related_name='provider')
For more information about reverse relations, refer to the Database API reference . For now, just remember to run manage.py validate when you’re writing your models and pay attention to the error messages.
As mentioned, Django will automatically create a OneToOneField linking your child class back any non-abstract parent models. If you want to control the name of the attribute linking back to the parent, you can create your own link field and pass it parent_link=True. For example, to explicitly specify the field that will link Supplier to Place in the above example, you could write:
class Supplier(Place):
parent = models.OneToOneField(Place, parent_link=True)
...
Just as with Python’s subclassing, it’s possible for a Django model to inherit from multiple parent models. Keep in mind that normal Python name resolution rules apply. The first base class that a particular name appears in (e.g. Meta) will be the one that is used. We stop searching once we find the name once. This means that if multiple parents contain a Meta class, only the first one is going to be used. All others will be ignored.
Generally, you won’t need to inherit from multiple parents. The main use-case where this is useful is for ‘’mix-in’’ classes: adding a particular extra field or method to every class that inherits the mix-in. Try to keep your inheritance hierarchies as simple and straightforward as possible so that you won’t have to struggle to work out where a particular piece of information is coming from.
It’s perfectly OK to relate a model to one from another app. To do this, just import the related model at the top of the model that holds your model. Then, just refer to the other model class wherever needed. For example:
from mysite.geography.models import ZipCode
class Restaurant(models.Model):
# ...
zip_code = models.ForeignKey(ZipCode)
Once you have created your models, the final step is to tell Django you’re going to use those models.
Do this by editing your settings file and changing the INSTALLED_APPS setting to add the name of the module that contains your models.py.
For example, if the models for your application live in the module mysite.myapp.models (the package structure that is created for an application by the manage.py startapp script), INSTALLED_APPS should read, in part:
INSTALLED_APPS = (
#...
'mysite.myapp',
#...
)
Django provides a hook for passing the database arbitrary SQL that’s executed just after the CREATE TABLE statements. Use this hook, for example, if you want to populate default records, or create SQL functions, automatically.
The hook is simple: Django just looks for a file called <appname>/sql/<modelname>.sql, where <appname> is your app directory and <modelname> is the model’s name in lowercase.
In the Person example model at the top of this document, assuming it lives in an app called myapp, you could add arbitrary SQL to the file myapp/sql/person.sql. Here’s an example of what the file might contain:
INSERT INTO myapp_person (first_name, last_name) VALUES ('John', 'Lennon');
INSERT INTO myapp_person (first_name, last_name) VALUES ('Paul', 'McCartney');
Each SQL file, if given, is expected to contain valid SQL. The SQL files are piped directly into the database after all of the models’ table-creation statements have been executed.
The SQL files are read by the sqlcustom, sqlreset, sqlall and reset commands in manage.py. Refer to the manage.py documentation for more information.
Note that if you have multiple SQL data files, there’s no guarantee of the order in which they’re executed. The only thing you can assume is that, by the time your custom data files are executed, all the database tables already will have been created.
There’s also a hook for backend-specific SQL data. For example, you can have separate initial-data files for PostgreSQL and MySQL. For each app, Django looks for a file called <appname>/sql/<modelname>.<backend>.sql, where <appname> is your app directory, <modelname> is the model’s name in lowercase and <backend> is the value of DATABASE_ENGINE in your settings file (e.g., postgresql, mysql).
Backend-specific SQL data is executed before non-backend-specific SQL data. For example, if your app contains the files sql/person.sql and sql/person.postgresql.sql and you’re installing the app on PostgreSQL, Django will execute the contents of sql/person.postgresql.sql first, then sql/person.sql.
If you notice errors with this documentation, please open a ticket and let us know!
Please only use the ticket tracker for criticisms and improvements on the docs. For tech support, ask in the IRC channel or post to the django-users list.