Fixed #24748 -- Fixed incorrect GROUP BY on MySQL in some queries

When the query's model had a self-referential foreign key, the
compiler.get_group_by() code incorrectly used the self-referential
foreign key's column (for example parent_id) as GROUP BY clause
when it should have used the model's primary key column (id).

django/db/models/sql/compiler.py

@@ -143,9 +143,12 @@ class SQLCompiler(object):
             # then also add having expressions to group by.
             pk = None
             for expr in expressions:
-                if (expr.output_field.primary_key and
-                        getattr(expr.output_field, 'model') == self.query.model):
+                # Is this a reference to query's base table primary key? If the
+                # expression isn't a Col-like, then skip the expression.
+                if (getattr(expr, 'target', None) == self.query.model._meta.pk and
+                        getattr(expr, 'alias', None) == self.query.tables[0]):
                     pk = expr
+                    break
             if pk:
                 # MySQLism: Columns in HAVING clause must be added to the GROUP BY.
                 expressions = [pk] + [expr for expr in expressions if expr in having]