Models ¶

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" bigint NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,
    "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.
Using models¶
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.
Fields¶
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()
Field types¶
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
How to create custom model fields.
Field options¶
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:
null
If True, Django will store empty values as NULL in the database.
Default is False.
blank
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, form validation will
allow entry of an empty value. If a field has blank=False, the field will be required.
choices
A sequence of 2-value tuples, a mapping, an
enumeration type, or a callable (that
expects no arguments and returns any of the previous formats), 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"),
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
>>> p.get_shirt_size_display()
'Large'
You can also use enumeration classes to define choices in a concise
from django.db import models
class Runner(models.Model):
    MedalType = models.TextChoices("MedalType", "GOLD SILVER BRONZE")
    name = models.CharField(max_length=60)
    medal = models.CharField(blank=True, choices=MedalType, max_length=10)
Further examples are available in the model field reference.
Changed in Django 5.0: 
Support for mappings and callables was added.
default
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.
help_text
Extra “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_key
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 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']>
unique
If 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.
Automatic primary key fields¶
By default, Django gives each model an auto-incrementing primary key with the
type specified per app in AppConfig.default_auto_field or globally in the
DEFAULT_AUTO_FIELD setting. For example:
id = models.BigAutoField(primary_key=True)
If you’d like to specify a custom primary key, 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).
Verbose field names¶
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.
Relationships¶
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.
Many-to-one relationships¶
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):
    # ...
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 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.
Many-to-many relationships¶
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):
    # ...
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.
Extra fields on many-to-many relationships¶
When you’re only dealing with 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.
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.'
One-to-one relationships¶
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.
Models across files¶
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,
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.
Custom field types¶
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 How to create 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.
Model attributes¶
objects
The 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.
Model methods¶
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 f"{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.
Overriding predefined model methods¶
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.
If you wish to update a field value in the save() method, you may
also want to have this field added to the update_fields keyword argument.
This will ensure the field is saved when update_fields is specified. For
example:
from django.db import models
from django.utils.text import slugify
class Blog(models.Model):
    name = models.CharField(max_length=100)
    slug = models.TextField()
    def save(
        self, force_insert=False, force_update=False, using=None, update_fields=None
        self.slug = slugify(self.name)
        if update_fields is not None and "name" in update_fields:
            update_fields = {"slug"}.union(update_fields)
        super().save(
            force_insert=force_insert,
            force_update=force_update,
            using=using,
            update_fields=update_fields,
See Specifying which fields to save for more details.
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.
Executing custom SQL¶
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¶
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¶
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. To make
an abstract base class that inherits from another abstract base class, you need
to explicitly set abstract=True on the child.
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.
Due to the way Python inheritance works, if a child class inherits from
multiple abstract base classes, only the Meta options
from the first listed class will be inherited by default. To inherit Meta options from multiple abstract base classes, you must
explicitly declare the Meta inheritance. For example:
from django.db import




    
 models
class CommonInfo(models.Model):
    name = models.CharField(max_length=100)
    age = models.PositiveIntegerField()
    class Meta:
        abstract = True
        ordering = ["name"]
class Unmanaged(models.Model):
    class Meta:
        abstract = True
        managed = False
class Student(CommonInfo, Unmanaged):
    home_group = models.CharField(max_length=5)
    class Meta(CommonInfo.Meta, Unmanaged.Meta):
Be careful with related_name and related_query_name¶
If you are using related_name or
related_query_name on a ForeignKey or
ManyToManyField, you must always specify a unique reverse name and query
name for the field. This would normally cause a problem in abstract base
classes, since the fields on this class are included into each of the child
classes, with exactly the same values for the attributes (including
related_name and
related_query_name) each time.
To work around this problem, when you are using
related_name or
related_query_name in an abstract base
class (only), part of the value should contain '%(app_label)s' and
'%(class)s'.
'%(class)s' is replaced by the lowercased name of the child class that
the field is used in.
'%(app_label)s' is replaced by the lowercased name of the app the child
class is contained within. Each installed application name must be unique and
the model class names within each app must also be unique, therefore the
resulting name will end up being different.
For example, given an app common/models.py:
from django.db import models
class Base(models.Model):
    m2m = models.ManyToManyField(
        OtherModel,
        related_name="%(app_label)s_%(class)s_related",
        related_query_name="%(app_label)s_%(class)ss",
    class Meta:
        abstract = True
class ChildA(Base):
class ChildB(Base):
Along with another app rare/models.py:
from common.models import Base
class ChildB(Base):
The reverse name of the common.ChildA.m2m field will be
common_childa_related and the reverse query name will be common_childas.
The reverse name of the common.ChildB.m2m field will be
common_childb_related and the reverse query name will be
common_childbs. Finally, the reverse name of the rare.ChildB.m2m field
will be rare_childb_related and the reverse query name will be
rare_childbs. It’s up to you how you use the '%(class)s' and
'%(app_label)s' portion to construct your related name or related query name
but if you forget to use it, Django will raise errors when you perform system
checks (or run migrate).
If you don’t specify a related_name
attribute for a field in an abstract base class, the default reverse name will
be the name of the child class followed by '_set', just as it normally
would be if you’d declared the field directly on the child class. For example,
in the above code, if the related_name
attribute was omitted, the reverse name for the m2m field would be
childa_set in the ChildA case and childb_set for the ChildB
field.
Multi-table inheritance¶
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,
    primary_key=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 = []
Inheritance and reverse relations¶
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').
Specifying the parent link field¶
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.
Proxy models¶
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):
        # ...
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: