======================
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 :class:`.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``:

.. code-block:: pycon

    >>> 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 :attr:`~django.db.models.Model.pk` attribute.
This sets the associated field values:

.. code-block:: pycon

    >>> 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``:

.. code-block:: pycon

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

We're still working on composite primary key support for
:ref:`relational fields <cpk-and-relations>`, including
:class:`.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, :class:`.SeparateDatabaseAndState` may be used to execute the
backend-specific migrations and Django-generated migrations in a single
operation.

.. _cpk-and-relations:

Composite primary keys and relations
====================================

:ref:`Relationship fields <relationship-fields>`, including
:ref:`generic relations <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.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.

.. _cpk-and-database-functions:

Composite primary keys and database functions
=============================================

Many database functions only accept a single expression.

.. code-block:: 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``.

.. code-block:: 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.

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:

.. code-block:: pycon

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

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

.. admonition:: 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
    :attr:`.Field.primary_key`).

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

Composite primary keys in model validation
==========================================

Since ``pk`` is only a virtual field, including ``pk`` as a field name in the
``exclude`` argument of :meth:`.Model.clean_fields` has no effect. To exclude
the composite primary key fields from
:ref:`model validation <validating-objects>`, specify each field individually.
:meth:`.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
:attr:`~django.db.models.Field.primary_key` attribute of its fields:

.. code-block:: pycon

    >>> 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
:attr:`~django.db.models.Field.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``:

.. code-block:: pycon

    >>> 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 :attr:`_meta.pk_fields <django.db.models.options.Options.pk_fields>`
attribute should be used instead:

.. code-block:: pycon

    >>> 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>
    ]
