Models¶

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):
    # ...
    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,
    )
    # ...

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):
    # ...
    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.

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 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.'

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.

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.

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.

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):
    pass

class ChildB(Base):
    pass

Along with another app rare/models.py:

from common.models import Base

class ChildB(Base):
    pass

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.

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.

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.

Base class restrictions¶

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.

Proxy model managers¶

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.

Differences between proxy inheritance and unmanaged models¶

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:

1. 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.
2. 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.