Refs #36042 -- Raised ValueError when providing composite expressions to aggregates.

docs/ref/models/expressions.txt

@@ -1105,6 +1105,14 @@ calling the appropriate methods on the wrapped expression.
         ``UNNEST``, etc.) to skip optimization and be properly evaluated when
         annotations spawn rows themselves. Defaults to ``False``.
 
+    .. attribute:: allows_composite_expressions
+
+        .. versionadded:: 5.2
+
+        Tells Django that this expression allows composite expressions, for
+        example, to support :ref:`composite primary keys
+        <cpk-and-database-functions>`. Defaults to ``False``.
+
     .. method:: resolve_expression(query=None, allow_joins=True, reuse=None, summarize=False, for_save=False)
 
         Provides the chance to do any preprocessing or validation of

docs/releases/5.2.txt

@@ -330,6 +330,10 @@ Models
   accepts a list of field names or expressions and returns a JSON array
   containing those values.
 
+* The new :attr:`.Expression.allows_composite_expressions` attribute specifies
+  that the expression allows composite expressions, for example, to support
+  :ref:`composite primary keys <cpk-and-database-functions>`.
+
 Requests and Responses
 ~~~~~~~~~~~~~~~~~~~~~~
 

docs/topics/composite-primary-key.txt

@@ -131,6 +131,8 @@ database.
     ``ForeignObject`` is an internal API. This means it is not covered by our
     :ref:`deprecation policy <internal-release-deprecation-policy>`.
 
+.. _cpk-and-database-functions:
+
 Composite primary keys and database functions
 =============================================
 
@@ -141,13 +143,15 @@ 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.
+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")  # ERROR
+    Max("pk")  # ValueError
+    Count("pk")  # OK
 
 Composite primary keys in forms
 ===============================