Models¶

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"),
]

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'

You can also use enumeration classes to define choices in a concise way:

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.

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.

db_default

The database-computed default value for the field. This can be a literal value or a database function.

If both db_default and Field.default are set, default will take precedence when creating instances in Python code. db_default will still be set at the database level and will be used when inserting rows outside of the ORM or when adding a new field in a migration.

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 a field 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.

from django.db import models


class Manufacturer(models.Model):
    # ...
    pass


class Car(models.Model):
    manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE)
    # ...

class Car(models.Model):
    company_that_makes_it = models.ForeignKey(
        Manufacturer,
        on_delete=models.CASCADE,
    )
    # ...

from django.db import models


class Topping(models.Model):
    # ...
    pass


class Pizza(models.Model):
    # ...
    toppings = models.ManyToManyField(Topping)

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)

    class Meta:
        constraints = [
            models.UniqueConstraint(
                fields=["person", "group"], name="unique_person_group"
            )
        ]

>>> 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>]>

>>> 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)}
... )

>>> 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>]>

>>> # Beatles have broken up
>>> beatles.members.clear()
>>> # Note that this deletes the intermediate model instances
>>> Membership.objects.all()
<QuerySet []>

# Find all the groups with a member whose name starts with 'Paul'
>>> Group.objects.filter(members__name__startswith="Paul")
<QuerySet [<Group: The Beatles>]>

# 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]>

>>> 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.'

>>> 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.'

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,
    )

Alternatively, you can use a lazy reference to the related model, specified as a string in the format "app_label.ModelName". This does not require the related model to be imported. For example:

from django.db import models


class Restaurant(models.Model):
    # ...
    zip_code = models.ForeignKey(
        "geography.ZipCode",
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

See lazy relationships for more details.

Field name restrictions¶

Django places some restrictions on model field names:

1. 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!
   

2. 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!
   

3. A field name cannot end with an underscore, for similar reasons.

4. A field name cannot be check, as this would override the check framework’s Model.check() method.

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.

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, **kwargs):
        do_something()
        super().save(**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, **kwargs):
        if self.name == "Yoko Ono's blog":
            return  # Yoko shall never have her own blog!
        else:
            super().save(**kwargs)  # Call the "real" save() method.

It’s important to remember to call the superclass method – that’s that super().save(**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 **kwargs bit does. Django will, from time to time, extend the capabilities of built-in model methods, adding new keyword arguments. If you use **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, **kwargs):
        self.slug = slugify(self.name)
        if (
            update_fields := kwargs.get("update_fields")
        ) is not None and "name" in update_fields:
            kwargs["update_fields"] = {"slug"}.union(update_fields)
        super().save(**kwargs)

See Specifying which fields to save for more details.

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.

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.

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"

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):
        pass

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):
    pass


class ChildB(Base):
    pass

from common.models import Base


class ChildB(Base):
    pass

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.

class ChildModel(ParentModel):
    # ...
    class Meta:
        # Remove parent's ordering effect
        ordering = []

class Supplier(Place):
    customers = models.ManyToManyField(Place)

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'.

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):
        # ...
        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:

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.

from django.db import models


class NewManager(models.Manager):
    # ...
    pass


class MyPerson(Person):
    objects = NewManager()

    class Meta:
        proxy = True

# 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

Multiple inheritance¶

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

Field name “hiding” is not permitted¶

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.

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.

Note that because of the way fields are resolved during class definition, model fields inherited from multiple abstract parent models are resolved in a strict depth-first order. This contrasts with standard Python MRO, which is resolved breadth-first in cases of diamond shaped inheritance. This difference only affects complex model hierarchies, which (as per the advice above) you should try to avoid.