A model is the single, definitive source of information 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:
Each model is a Python class that subclasses
django.db.models.Model.
Each attribute of the model represents a database field.
With all of this, Django gives you an automatically-generated database-access API; see Making queries.
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 name of the table, myapp_person, is automatically derived from
some model metadata but can be overridden. See Table names for more
details.
An id field is added automatically, but this behavior can be
overridden. See Automatic primary key fields.
The CREATE TABLE SQL in this example is formatted using PostgreSQL
syntax, but it’s worth noting Django uses SQL tailored to the database
backend specified in your settings file.
Once you have defined your models, you need 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
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 = [
#...
'myapp',
#...
]
When you add new apps to INSTALLED_APPS, be sure to run
manage.py migrate, optionally making migrations
for them first with manage.py makemigrations.
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. Be careful not to choose field names that conflict with the
models API like clean, save, or
delete.
Example:
from django.db import models
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, on_delete=models.CASCADE)
name = models.CharField(max_length=100)
release_date = models.DateField()
num_stars = models.IntegerField()
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:
The column type, which tells the database what kind of data to store (e.g.
INTEGER, VARCHAR, TEXT).
The default HTML widget to use when rendering a form
field (e.g. <input type="text">, <select>).
The minimal validation requirements, used in Django’s admin and in automatically-generated forms.
Django ships with dozens of built-in field types; you can find the complete list in the model field reference. You can easily write your own fields if Django’s built-in ones don’t do the trick; see Writing custom model fields.
Each field takes a certain set of field-specific arguments (documented in the
model field reference). For example,
CharField (and its subclasses) require a
max_length argument which specifies the size
of the VARCHAR database field used to store the data.
There’s also a set of common arguments available to all field types. All are optional. They’re fully explained in the reference, but here’s a quick summary of the most often-used ones:
nullIf True, Django will store empty values as NULL in the database.
Default is False.
blankIf 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, form validation will
allow entry of an empty value. If a field has blank=False, the field will be required.
choicesA sequence of 2-tuples to use as choices for this field. If this is given, the default form widget will be 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'),
]
Note
A new migration is created each time the order of choices changes.
The first element in each tuple is the value that will be stored in the database. The second element is displayed by the field’s form widget.
Given a model instance, the display value for a field with choices can
be accessed using the get_FOO_display()
method. For example:
from django.db import models
class Person(models.Model):
SHIRT_SIZES = (
('S', 'Small'),
('M', 'Medium'),
('L', 'Large'),
)
name = models.CharField(max_length=60)
shirt_size = models.CharField(max_length=1, choices=SHIRT_SIZES)
>>> p = Person(name="Fred Flintstone", shirt_size="L")
>>> p.save()
>>> p.shirt_size
'L'
>>> p.get_shirt_size_display()
'Large'
defaultThe 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.
help_textExtra “help” text to be displayed with the form widget. It’s useful for documentation even if your field isn’t used on a form.
primary_keyIf 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 an
IntegerField to hold the primary key, so you don’t need to set
primary_key=True on any of your fields
unless you want to override the default primary-key behavior. For more,
see Automatic primary key fields.
The primary key field is read-only. If you change the value of the primary key on an existing object and then save it, a new object will be created alongside the old one. For example:
from django.db import models
class Fruit(models.Model):
name = models.CharField(max_length=100, primary_key=True)
>>> fruit = Fruit.objects.create(name='Apple')
>>> fruit.name = 'Pear'
>>> fruit.save()
>>> Fruit.objects.values_list('name', flat=True)
<QuerySet ['Apple', 'Pear']>
uniqueIf True, this field must be unique throughout the table.
Again, these are just short descriptions of the most common field options. Full details can be found in the common model field option reference.
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 Field.primary_key, it won’t add the automatic
id column.
Each model requires exactly one field to have primary_key=True (either explicitly declared or automatically added).
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,
on_delete=models.CASCADE,
verbose_name="the related poll",
)
sites = models.ManyToManyField(Site, verbose_name="list of sites")
place = models.OneToOneField(
Place,
on_delete=models.CASCADE,
verbose_name="related place",
)
The 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 django.db.models.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:
from django.db import models
class Manufacturer(models.Model):
# ...
pass
class Car(models.Model):
manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
# ...
You can also create recursive relationships (an object with a many-to-one relationship to itself) and relationships to models not yet defined; see the model field reference for details.
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,
on_delete=models.CASCADE,
)
# ...
See also
ForeignKey fields accept a number of extra
arguments which are explained in the model field reference. These options help define how the relationship
should work; all are optional.
For details on accessing backwards-related objects, see the Following relationships backward example.
For sample code, see the Many-to-one relationship model example.
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:
from django.db import models
class Topping(models.Model):
# ...
pass
class Pizza(models.Model):
# ...
toppings = models.ManyToManyField(Topping)
As with ForeignKey, you can also create
recursive relationships (an object with a
many-to-many relationship to itself) and relationships to models not yet
defined.
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.
It doesn’t matter which model has the
ManyToManyField, but you should only put it in one
of the models – not both.
Generally, ManyToManyField instances should go in
the object that’s going to be edited on a form. 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 form would let users select the toppings.
See also
See the Many-to-many relationship model example for a full example.
ManyToManyField fields also accept a number of
extra arguments which are explained in the model field reference. These options help define how the relationship
should work; all are optional.
When you’re only dealing with simple many-to-many relationships such as
mixing and matching pizzas and toppings, a standard
ManyToManyField is all you need. However, sometimes
you may need to associate data with the relationship between two models.
For example, consider the case of an application tracking the musical groups
which musicians belong to. There is a many-to-many relationship between a person
and the groups of which they are a member, so you could use a
ManyToManyField to represent this relationship.
However, there is a lot of detail about the membership that you might want to
collect, such as the date at which the person joined the group.
For these situations, Django allows you to specify the model that will be used
to govern the many-to-many relationship. You can then put extra fields on the
intermediate model. The intermediate model is associated with the
ManyToManyField using the
through argument to point to the model
that will act as an intermediary. For our musician example, the code would look
something like this:
from django.db import models
class Person(models.Model):
name = models.CharField(max_length=128)
def __str__(self):
return self.name
class Group(models.Model):
name = models.CharField(max_length=128)
members = models.ManyToManyField(Person, through='Membership')
def __str__(self):
return self.name
class Membership(models.Model):
person = models.ForeignKey(Person, on_delete=models.CASCADE)
group = models.ForeignKey(Group, on_delete=models.CASCADE)
date_joined = models.DateField()
invite_reason = models.CharField(max_length=64)
When you set up the intermediary model, you explicitly specify foreign keys to the models that are involved in the many-to-many relationship. This explicit declaration defines how the two models are related.
There are a few restrictions on the intermediate model:
Your intermediate model must contain one - and only one - foreign key
to the source model (this would be Group in our example), or you must
explicitly specify the foreign keys Django should use for the relationship
using ManyToManyField.through_fields.
If you have more than one foreign key and through_fields is not
specified, a validation error will be raised. A similar restriction applies
to the foreign key to the target model (this would be Person in our
example).
For a model which has a many-to-many relationship to itself through an
intermediary model, two foreign keys to the same model are permitted, but
they will be treated as the two (different) sides of the many-to-many
relationship. If there are more than two foreign keys though, you
must also specify through_fields as above, or a validation error
will be raised.
When defining a many-to-many relationship from a model to
itself, using an intermediary model, you must use
symmetrical=False (see
the model field reference).
Now that you have set up your ManyToManyField to use
your intermediary model (Membership, in this case), you’re ready to start
creating some many-to-many relationships. You do this by creating instances of
the intermediate model:
>>> ringo = Person.objects.create(name="Ringo Starr")
>>> paul = Person.objects.create(name="Paul McCartney")
>>> beatles = Group.objects.create(name="The Beatles")
>>> m1 = Membership(person=ringo, group=beatles,
... date_joined=date(1962, 8, 16),
... invite_reason="Needed a new drummer.")
>>> m1.save()
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>]>
>>> ringo.group_set.all()
<QuerySet [<Group: The Beatles>]>
>>> m2 = Membership.objects.create(person=paul, group=beatles,
... date_joined=date(1960, 8, 1),
... invite_reason="Wanted to form a band.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>]>
You can also use add(), create(), or set() to create relationships,
as long as you specify through_defaults for any required fields:
>>> beatles.members.add(john, through_defaults={'date_joined': date(1960, 8, 1)})
>>> beatles.members.create(name="George Harrison", through_defaults={'date_joined': date(1960, 8, 1)})
>>> beatles.members.set([john, paul, ringo, george], through_defaults={'date_joined': date(1960, 8, 1)})
You may prefer to create instances of the intermediate model directly.
If the custom through table defined by the intermediate model does not enforce
uniqueness on the (model1, model2) pair, allowing multiple values, the
remove() call will
remove all intermediate model instances:
>>> Membership.objects.create(person=ringo, group=beatles,
... date_joined=date(1968, 9, 4),
... invite_reason="You've been gone for a month and we miss you.")
>>> beatles.members.all()
<QuerySet [<Person: Ringo Starr>, <Person: Paul McCartney>, <Person: Ringo Starr>]>
>>> # This deletes both of the intermediate model instances for Ringo Starr
>>> beatles.members.remove(ringo)
>>> beatles.members.all()
<QuerySet [<Person: Paul McCartney>]>
The clear()
method can be used to remove all many-to-many relationships for an instance:
>>> # Beatles have broken up
>>> beatles.members.clear()
>>> # Note that this deletes the intermediate model instances
>>> Membership.objects.all()
<QuerySet []>
Once you have established the many-to-many relationships, you can issue queries. Just as with normal many-to-many relationships, you can query using the attributes of the many-to-many-related model:
# Find all the groups with a member whose name starts with 'Paul'
>>> Group.objects.filter(members__name__startswith='Paul')
<QuerySet [<Group: The Beatles>]>
As you are using an intermediate model, you can also query on its attributes:
# Find all the members of the Beatles that joined after 1 Jan 1961
>>> Person.objects.filter(
... group__name='The Beatles',
... membership__date_joined__gt=date(1961,1,1))
<QuerySet [<Person: Ringo Starr]>
If you need to access a membership’s information you may do so by directly
querying the Membership model:
>>> ringos_membership = Membership.objects.get(group=beatles, person=ringo)
>>> ringos_membership.date_joined
datetime.date(1962, 8, 16)
>>> ringos_membership.invite_reason
'Needed a new drummer.'
Another way to access the same information is by querying the
many-to-many reverse relationship from a
Person object:
>>> ringos_membership = ringo.membership_set.get(group=beatles)
>>> ringos_membership.date_joined
datetime.date(1962, 8, 16)
>>> ringos_membership.invite_reason
'Needed a new drummer.'
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 were 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; in fact, to handle this you’d typically use
inheritance, which involves an implicit
one-to-one relation).
As with ForeignKey, a recursive relationship can be defined and references to as-yet
undefined models can be made.
See also
See the One-to-one relationship model example for a full example.
OneToOneField fields also accept an optional
parent_link argument.
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 argument if you like).
Thus, it’s now possible to have multiple fields of type
OneToOneField on a single model.
It’s perfectly OK to relate a model to one from another app. To do this, import the related model at the top of the file where your model is defined. Then, just refer to the other model class wherever needed. For example:
from django.db import models
from geography.models import ZipCode
class Restaurant(models.Model):
# ...
zip_code = models.ForeignKey(
ZipCode,
on_delete=models.SET_NULL,
blank=True,
null=True,
)
Django places some 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!
A field name cannot end with an underscore, for similar reasons.
These limitations can be worked around, though, because your field name doesn’t
necessarily have to match your database column name. See the
db_column option.
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.
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 Writing custom model fields.
Meta options¶Give your model metadata by using an inner class Meta, like so:
from django.db import models
class Ox(models.Model):
horn_length = models.IntegerField()
class Meta:
ordering = ["horn_length"]
verbose_name_plural = "oxen"
Model metadata is “anything that’s not a field”, such as ordering options
(ordering), database table name (db_table), or
human-readable singular and plural names (verbose_name and
verbose_name_plural). None are required, and adding class
Meta to a model is completely optional.
A complete list of all possible Meta options can be found in the model
option reference.
objectsThe most important attribute of a model is the
Manager. It’s the interface through which
database query operations are provided to Django models and is used to
retrieve the instances from the database. If no
custom Manager is defined, the default name is
objects. Managers are only accessible via
model classes, not the model instances.
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:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=50)
last_name = models.CharField(max_length=50)
birth_date = models.DateField()
def baby_boomer_status(self):
"Returns the person's baby-boomer status."
import datetime
if self.birth_date < datetime.date(1945, 8, 1):
return "Pre-boomer"
elif self.birth_date < datetime.date(1965, 1, 1):
return "Baby boomer"
else:
return "Post-boomer"
@property
def full_name(self):
"Returns the person's full name."
return '%s %s' % (self.first_name, self.last_name)
The last method in this example is a property.
The model instance reference has a complete list of methods automatically given to each model. You can override most of these – see overriding predefined model methods, below – but there are a couple that you’ll almost always want to define:
__str__()A Python “magic method” that returns a string representation of any object. This is what Python and Django will use whenever a model instance needs to be coerced and displayed as a plain string. Most notably, this happens when you display an object in an interactive console or in the admin.
You’ll always want to define this method; the default isn’t very helpful at all.
get_absolute_url()This tells Django how to calculate the URL for an object. Django uses this in its admin interface, and any time it needs to figure out a URL for an object.
Any object that has a URL that uniquely identifies it should define this method.
There’s another set of model methods that
encapsulate a bunch of database behavior that you’ll want to customize. In
particular you’ll often want to change the way save() and
delete() work.
You’re free to override these methods (and any other model method) 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 (see
save() for documentation of the parameters it accepts):
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self, *args, **kwargs):
do_something()
super().save(*args, **kwargs) # Call the "real" save() method.
do_something_else()
You can also prevent saving:
from django.db import models
class Blog(models.Model):
name = models.CharField(max_length=100)
tagline = models.TextField()
def save(self, *args, **kwargs):
if self.name == "Yoko Ono's blog":
return # Yoko shall never have her own blog!
else:
super().save(*args, **kwargs) # Call the "real" save() method.
It’s important to remember to call the superclass method – that’s
that super().save(*args, **kwargs) business – to ensure
that the object still gets saved into the database. If you forget to
call the superclass method, the default behavior won’t happen and the
database won’t get touched.
It’s also important that you pass through the arguments that can be
passed to the model method – that’s what the *args, **kwargs bit
does. Django will, from time to time, extend the capabilities of
built-in model methods, adding new arguments. If you use *args,
**kwargs in your method definitions, you are guaranteed that your
code will automatically support those arguments when they are added.
Overridden model methods are not called on bulk operations
Note that the delete() method for an object is not
necessarily called when deleting objects in bulk using a
QuerySet or as a result of a cascading
delete. To ensure customized
delete logic gets executed, you can use
pre_delete and/or
post_delete signals.
Unfortunately, there isn’t a workaround when
creating or
updating objects in bulk,
since none of save(),
pre_save, and
post_save are called.
Another common pattern is writing custom SQL statements in model methods and module-level methods. For more details on using raw SQL, see the documentation on using raw SQL.
Model inheritance in Django works almost identically to the way normal
class inheritance works in Python, but the basics at the beginning of the page
should still be followed. That means the base class should subclass
django.db.models.Model.
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.
There are three styles of inheritance that are possible in Django.
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.
If you’re subclassing an existing model (perhaps something from another application entirely) and want each model to have its own database table, Multi-table inheritance is the way to go.
Finally, if you only want to modify the Python-level behavior of a model, without changing the models fields in any way, you can use Proxy models.
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.
An example:
from django.db import models
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, and cannot be instantiated or saved directly.
Fields inherited from abstract base classes can be overridden with another
field or value, or be removed with None.
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, while still only creating one database table per child model at the database level.
Meta inheritance¶When an abstract base class is created, Django makes any Meta inner class you declared in 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:
from django.db import models
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:
from django.db import models
class Place(models.Model):
name = models.CharField(max_length=50)
address = models.CharField(max_length=80)
class Restaurant(Place):
serves_hot_dogs = models.BooleanField(default=False)
serves_pizza = models.BooleanField(default=False)
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 lowercase version of
the model name:
>>> p = Place.objects.get(id=12)
# If p 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 raise a Restaurant.DoesNotExist
exception.
The automatically-created OneToOneField on
Restaurant that links it to Place looks like this:
place_ptr = models.OneToOneField(
Place, on_delete=models.CASCADE,
parent_link=True,
)
You can override that field by declaring your own
OneToOneField with parent_link=True on Restaurant.
Meta and multi-table inheritance¶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 behavior (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 behavior 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 disable it:
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 types of relations on a subclass of the parent model, you
must specify the related_name
attribute on each such field. If you forget, Django will raise a validation
error.
For example, using the above Place class again, let’s create another
subclass with a ManyToManyField:
class Supplier(Place):
customers = models.ManyToManyField(Place)
This results in the error:
Reverse query name for 'Supplier.customers' clashes with reverse query
name for 'Supplier.place_ptr'.
HINT: Add or change a related_name argument to the definition for
'Supplier.customers' or 'Supplier.place_ptr'.
Adding related_name to the customers field as follows would resolve the
error: models.ManyToManyField(Place, related_name='provider').
As mentioned, Django will automatically create a
OneToOneField linking your child
class back to 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 OneToOneField and set
parent_link=True
to indicate that your field is the link back to the parent class.
When using multi-table inheritance, a new database table is created for each subclass of a model. This is usually the desired behavior, since the subclass needs a place to store any additional data fields that are not present on the base class. Sometimes, however, you only want to change the Python behavior of a model – perhaps to change the default manager, or add a new method.
This is what proxy model inheritance is for: creating a proxy for the original model. You can create, delete and update instances of the proxy model and all the data will be saved as if you were using the original (non-proxied) model. The difference is that you can change things like the default model ordering or the default manager in the proxy, without having to alter the original.
Proxy models are declared like normal models. You tell Django that it’s a
proxy model by setting the proxy attribute of
the Meta class to True.
For example, suppose you want to add a method to the Person model. You can do it like this:
from django.db import models
class Person(models.Model):
first_name = models.CharField(max_length=30)
last_name = models.CharField(max_length=30)
class MyPerson(Person):
class Meta:
proxy = True
def do_something(self):
# ...
pass
The MyPerson class operates on the same database table as its parent
Person class. In particular, any new instances of Person will also be
accessible through MyPerson, and vice-versa:
>>> p = Person.objects.create(first_name="foobar")
>>> MyPerson.objects.get(first_name="foobar")
<MyPerson: foobar>
You could also use a proxy model to define a different default ordering on
a model. You might not always want to order the Person model, but regularly
order by the last_name attribute when you use the proxy. This is easy:
class OrderedPerson(Person):
class Meta:
ordering = ["last_name"]
proxy = True
Now normal Person queries will be unordered
and OrderedPerson queries will be ordered by last_name.
Proxy models inherit Meta attributes in the same way as regular
models.
QuerySets still return the model that was requested¶There is no way to have Django return, say, a MyPerson object whenever you
query for Person objects. A queryset for Person objects will return
those types of objects. The whole point of proxy objects is that code relying
on the original Person will use those and your own code can use the
extensions you included (that no other code is relying on anyway). It is not
a way to replace the Person (or any other) model everywhere with something
of your own creation.
A proxy model must inherit from exactly one non-abstract model class. You can’t inherit from multiple non-abstract models as the proxy model doesn’t provide any connection between the rows in the different database tables. A proxy model can inherit from any number of abstract model classes, providing they do not define any model fields. A proxy model may also inherit from any number of proxy models that share a common non-abstract parent class.
If you don’t specify any model managers on a proxy model, it inherits the managers from its model parents. If you define a manager on the proxy model, it will become the default, although any managers defined on the parent classes will still be available.
Continuing our example from above, you could change the default manager used
when you query the Person model like this:
from django.db import models
class NewManager(models.Manager):
# ...
pass
class MyPerson(Person):
objects = NewManager()
class Meta:
proxy = True
If you wanted to add a new manager to the Proxy, without replacing the existing default, you can use the techniques described in the custom manager documentation: create a base class containing the new managers and inherit that after the primary base class:
# Create an abstract class for the new manager.
class ExtraManagers(models.Model):
secondary = NewManager()
class Meta:
abstract = True
class MyPerson(Person, ExtraManagers):
class Meta:
proxy = True
You probably won’t need to do this very often, but, when you do, it’s possible.
Proxy model inheritance might look fairly similar to creating an unmanaged
model, using the managed attribute on a
model’s Meta class.
With careful setting of Meta.db_table you could create an unmanaged model that
shadows an existing model and adds Python methods to it. However, that would be
very repetitive and fragile as you need to keep both copies synchronized if you
make any changes.
On the other hand, proxy models are intended to behave exactly like the model they are proxying for. They are always in sync with the parent model since they directly inherit its fields and managers.
The general rules are:
If you are mirroring an existing model or database table and don’t want
all the original database table columns, use Meta.managed=False.
That option is normally useful for modeling database views and tables
not under the control of Django.
If you are wanting to change the Python-only behavior of a model, but
keep all the same fields as in the original, use Meta.proxy=True.
This sets things up so that the proxy model is an exact copy of the
storage structure of the original model when data is saved.
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 (e.g. Meta) appears in will be the one that is used; for example, this means that if multiple parents contain a Meta class, only the first one is going to be used, and 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.
Note that inheriting from multiple models that have a common id primary
key field will raise an error. To properly use multiple inheritance, you can
use an explicit AutoField in the base models:
class Article(models.Model):
article_id = models.AutoField(primary_key=True)
...
class Book(models.Model):
book_id = models.AutoField(primary_key=True)
...
class BookReview(Book, Article):
pass
Or use a common ancestor to hold the AutoField. This
requires using an explicit OneToOneField from each
parent model to the common ancestor to avoid a clash between the fields that
are automatically generated and inherited by the child:
class Piece(models.Model):
pass
class Article(Piece):
article_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
...
class Book(Piece):
book_piece = models.OneToOneField(Piece, on_delete=models.CASCADE, parent_link=True)
...
class BookReview(Book, Article):
pass
In normal Python class inheritance, it is permissible for a child class to
override any attribute from the parent class. In Django, this isn’t usually
permitted for model fields. If a non-abstract model base class has a field
called author, you can’t create another model field or define
an attribute called author in any class that inherits from that base class.
This restriction doesn’t apply to model fields inherited from an abstract
model. Such fields may be overridden with another field or value, or be removed
by setting field_name = None.
Warning
Model managers are inherited from abstract base classes. Overriding an
inherited field which is referenced by an inherited
Manager may cause subtle bugs. See custom
managers and model inheritance.
Note
Some fields define extra attributes on the model, e.g. a
ForeignKey defines an extra attribute with
_id appended to the field name, as well as related_name and
related_query_name on the foreign model.
These extra attributes cannot be overridden unless the field that defines it is changed or removed so that it no longer defines the extra attribute.
Overriding fields in a parent model leads to difficulties in areas such as
initializing new instances (specifying which field is being initialized in
Model.__init__) and serialization. These are features which normal Python
class inheritance doesn’t have to deal with in quite the same way, so the
difference between Django model inheritance and Python class inheritance isn’t
arbitrary.
This restriction only applies to attributes which are
Field instances. Normal Python attributes
can be overridden if you wish. It also only applies to the name of the
attribute as Python sees it: if you are manually specifying the database
column name, you can have the same column name appearing in both a child and
an ancestor model for multi-table inheritance (they are columns in two
different database tables).
Django will raise a FieldError if you override
any model field in any ancestor model.
The manage.py startapp command creates an application
structure that includes a models.py file. If you have many models,
organizing them in separate files may be useful.
To do so, create a models package. Remove models.py and create a
myapp/models/ directory with an __init__.py file and the files to
store your models. You must import the models in the __init__.py file.
For example, if you had organic.py and synthetic.py in the models
directory:
from .organic import Person
from .synthetic import Robot
Explicitly importing each model rather than using from .models import *
has the advantages of not cluttering the namespace, making code more readable,
and keeping code analysis tools useful.
See also
Covers all the model related APIs including model fields, related
objects, and QuerySet.
Dec 25, 2023