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Composite primary keys

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 defining a model set the pk attribute to be a CompositePrimaryKey:

Code
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:

Python console
>>> 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 the pk attribute. This sets the associated field values:

Python console
>>> 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:

Python console
>>> 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:

Code
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:

Code
class Foo(models.Model):
    item_order_id = models.CharField(max_length=20)
    item_product_id = models.IntegerField()
    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, and the on_delete argument is ignored.

Composite primary keys and database functions

Many database functions only accept a single expression.

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

In these cases, providing a composite primary key reference raises a ValueError, since it is composed of multiple column expressions. An exception is made for Count.

Python
Max("order_id")  # OK
Max("pk")  # ValueError
Count("pk")  # OK

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.

例如，采取以下表单：

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

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

Python console
>>> 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.

Composite primary keys in model validation

Since pk is only a virtual field, including pk as a field name in the exclude argument of Model.clean_fields() has no effect. To exclude the composite primary key fields from model validation, specify each field individually. Model.validate_unique() can still be called with exclude={"pk"} to skip uniqueness checks.

Building composite primary key ready applications

Prior to the introduction of composite primary keys, the single field composing the primary key of a model could be retrieved by introspecting the primary_key attribute of its fields:

Python console
>>> pk_field = None
>>> for field in Product._meta.get_fields():
...     if field.primary_key:
...         pk_field = field
...         break
...
>>> pk_field
<django.db.models.fields.AutoField: id>

Now that a primary key can be composed of multiple fields the primary_key attribute can no longer be relied upon to identify members of the primary key as it will be set to False to maintain the invariant that at most one field per model will have this attribute set to True:

Python console
>>> pk_fields = []
>>> for field in OrderLineItem._meta.get_fields():
...     if field.primary_key:
...         pk_fields.append(field)
...
>>> pk_fields
[]

In order to build application code that properly handles composite primary keys the _meta.pk_fields attribute should be used instead:

Python console
>>> Product._meta.pk_fields
[<django.db.models.fields.AutoField: id>]
>>> OrderLineItem._meta.pk_fields
[
    <django.db.models.fields.ForeignKey: product>,
    <django.db.models.fields.ForeignKey: order>
]

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