Fixed #373 -- Added CompositePrimaryKey.

Thanks Lily Foote and Simon Charette for reviews and mentoring
this Google Summer of Code 2024 project.

Co-authored-by: Simon Charette <charette.s@gmail.com>
Co-authored-by: Lily Foote <code@lilyf.org>

docs/ref/checks.txt

@@ -181,6 +181,7 @@ Model fields
 * **fields.E011**: ``<database>`` does not support default database values with
   expressions (``db_default``).
 * **fields.E012**: ``<expression>`` cannot be used in ``db_default``.
+* **fields.E013**: ``CompositePrimaryKey`` must be named ``pk``.
 * **fields.E100**: ``AutoField``\s must set primary_key=True.
 * **fields.E110**: ``BooleanField``\s do not accept null values. *This check
   appeared before support for null values was added in Django 2.1.*
@@ -417,6 +418,8 @@ Models
 * **models.W040**: ``<database>`` does not support indexes with non-key
   columns.
 * **models.E041**: ``constraints`` refers to the joined field ``<field name>``.
+* **models.E042**: ``<field name>`` cannot be included in the composite
+  primary key.
 * **models.W042**: Auto-created primary key used when not defining a primary
   key type, by default ``django.db.models.AutoField``.
 * **models.W043**: ``<database>`` does not support indexes on expressions.

docs/ref/models/fields.txt

@@ -707,6 +707,23 @@ or :class:`~django.forms.NullBooleanSelect` if :attr:`null=True <Field.null>`.
 The default value of ``BooleanField`` is ``None`` when :attr:`Field.default`
 isn't defined.
 
+``CompositePrimaryKey``
+-----------------------
+
+.. versionadded:: 5.2
+
+.. class:: CompositePrimaryKey(*field_names, **options)
+
+A virtual field used for defining a composite primary key.
+
+This field must be defined as the model's ``pk`` field. If present, Django will
+create the underlying model table with a composite primary key.
+
+The ``*field_names`` argument is a list of positional field names that compose
+the primary key.
+
+See :doc:`/topics/composite-primary-key` for more details.
+
 ``CharField``
 -------------
 
@@ -1615,6 +1632,8 @@ not an instance of ``UUID``.
     hyphens, because PostgreSQL and MariaDB 10.7+ store them in a hyphenated
     uuid datatype type.
 
+.. _relationship-fields:
+
 Relationship fields
 ===================