• Language: en
  • Documentation version: development

Composite primary keys

New in Django Development version.

In Django, each model has a primary key. By default, this primary key consists of a single field.

In most cases, a single primary key should suffice. In database design, however, defining a primary key consisting of multiple fields is sometimes necessary.

To use a composite primary key, when creating a model set the pk field to be a CompositePrimaryKey:

class Product(models.Model):
    name = models.CharField(max_length=100)


class Order(models.Model):
    reference = models.CharField(max_length=20, primary_key=True)


class OrderLineItem(models.Model):
    pk = models.CompositePrimaryKey("product_id", "order_id")
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    order = models.ForeignKey(Order, on_delete=models.CASCADE)
    quantity = models.IntegerField()

This will instruct Django to create a composite primary key (PRIMARY KEY (product_id, order_id)) when creating the table.

A composite primary key is represented by a tuple:

>>> product = Product.objects.create(name="apple")
>>> order = Order.objects.create(reference="A755H")
>>> item = OrderLineItem.objects.create(product=product, order=order, quantity=1)
>>> item.pk
(1, "A755H")

You can assign a tuple to a composite primary key. This sets the associated field values.

>>> item = OrderLineItem(pk=(2, "B142C"))
>>> item.pk
(2, "B142C")
>>> item.product_id
2
>>> item.order_id
"B142C"

A composite primary key can also be filtered by a tuple:

>>> OrderLineItem.objects.filter(pk=(1, "A755H")).count()
1

We’re still working on composite primary key support for relational fields, including GenericForeignKey fields, and the Django admin. Models with composite primary keys cannot be registered in the Django admin at this time. You can expect to see this in future releases.

Migrating to a composite primary key

Django doesn’t support migrating to, or from, a composite primary key after the table is created. It also doesn’t support adding or removing fields from the composite primary key.

If you would like to migrate an existing table from a single primary key to a composite primary key, follow your database backend’s instructions to do so.

Once the composite primary key is in place, add the CompositePrimaryKey field to your model. This allows Django to recognize and handle the composite primary key appropriately.

While migration operations (e.g. AddField, AlterField) on primary key fields are not supported, makemigrations will still detect changes.

In order to avoid errors, it’s recommended to apply such migrations with --fake.

Alternatively, SeparateDatabaseAndState may be used to execute the backend-specific migrations and Django-generated migrations in a single operation.

Composite primary keys and relations

Relationship fields, including generic relations do not support composite primary keys.

For example, given the OrderLineItem model, the following is not supported:

class Foo(models.Model):
    item = models.ForeignKey(OrderLineItem, on_delete=models.CASCADE)

Because ForeignKey currently cannot reference models with composite primary keys.

To work around this limitation, ForeignObject can be used as an alternative:

class Foo(models.Model):
    item_order_id = models.IntegerField()
    item_product_id = models.CharField(max_length=20)
    item = models.ForeignObject(
        OrderLineItem,
        on_delete=models.CASCADE,
        from_fields=("item_order_id", "item_product_id"),
        to_fields=("order_id", "product_id"),
    )

ForeignObject is much like ForeignKey, except that it doesn’t create any columns (e.g. item_id), foreign key constraints or indexes in the database.

Warning

ForeignObject is an internal API. This means it is not covered by our deprecation policy.

Composite primary keys and database functions

Many database functions only accept a single expression.

MAX("order_id")  -- OK
MAX("product_id", "order_id")  -- ERROR

As a consequence, they cannot be used with composite primary key references as they are composed of multiple column expressions.

Max("order_id")  # OK
Max("pk")  # ERROR

Composite primary keys in forms

As a composite primary key is a virtual field, a field which doesn’t represent a single database column, this field is excluded from ModelForms.

For example, take the following form:

class OrderLineItemForm(forms.ModelForm):
    class Meta:
        model = OrderLineItem
        fields = "__all__"

This form does not have a form field pk for the composite primary key:

>>> OrderLineItemForm()
<OrderLineItemForm bound=False, valid=Unknown, fields=(product;order;quantity)>

Setting the primary composite field pk as a form field raises an unknown field FieldError.

Primary key fields are read only

If you change the value of a primary key on an existing object and then save it, a new object will be created alongside the old one (see Field.primary_key).

This is also true of composite primary keys. Hence, you may want to set Field.editable to False on all primary key fields to exclude them from ModelForms.

Back to Top