Fixed #27995 -- Added error messages on unsupported operations following union(), intersection(), and difference().

django/db/models/query.py

@@ -401,7 +401,7 @@ class QuerySet:
         Perform the query and return a single object matching the given
         keyword arguments.
         """
-        clone = self.filter(*args, **kwargs)
+        clone = self._chain() if self.query.combinator else self.filter(*args, **kwargs)
         if self.query.can_filter() and not self.query.distinct_fields:
             clone = clone.order_by()
         limit = None
@@ -890,6 +890,7 @@ class QuerySet:
         Return a new QuerySet instance with the args ANDed to the existing
         set.
         """
+        self._not_support_combined_queries('filter')
         return self._filter_or_exclude(False, *args, **kwargs)
 
     def exclude(self, *args, **kwargs):
@@ -897,6 +898,7 @@ class QuerySet:
         Return a new QuerySet instance with NOT (args) ANDed to the existing
         set.
         """
+        self._not_support_combined_queries('exclude')
         return self._filter_or_exclude(True, *args, **kwargs)
 
     def _filter_or_exclude(self, negate, *args, **kwargs):
@@ -985,7 +987,7 @@ class QuerySet:
 
         If select_related(None) is called, clear the list.
         """
-
+        self._not_support_combined_queries('select_related')
         if self._fields is not None:
             raise TypeError("Cannot call select_related() after .values() or .values_list()")
 
@@ -1007,6 +1009,7 @@ class QuerySet:
         When prefetch_related() is called more than once, append to the list of
         prefetch lookups. If prefetch_related(None) is called, clear the list.
         """
+        self._not_support_combined_queries('prefetch_related')
         clone = self._chain()
         if lookups == (None,):
             clone._prefetch_related_lookups = ()
@@ -1025,6 +1028,7 @@ class QuerySet:
         Return a query set in which the returned objects have been annotated
         with extra data or aggregations.
         """
+        self._not_support_combined_queries('annotate')
         self._validate_values_are_expressions(args + tuple(kwargs.values()), method_name='annotate')
         annotations = {}
         for arg in args:
@@ -1088,6 +1092,7 @@ class QuerySet:
     def extra(self, select=None, where=None, params=None, tables=None,
               order_by=None, select_params=None):
         """Add extra SQL fragments to the query."""
+        self._not_support_combined_queries('extra')
         assert self.query.can_filter(), \
             "Cannot change a query once a slice has been taken"
         clone = self._chain()
@@ -1109,6 +1114,7 @@ class QuerySet:
         The only exception to this is if None is passed in as the only
         parameter, in which case removal all deferrals.
         """
+        self._not_support_combined_queries('defer')
         if self._fields is not None:
             raise TypeError("Cannot call defer() after .values() or .values_list()")
         clone = self._chain()
@@ -1124,6 +1130,7 @@ class QuerySet:
         method and that are not already specified as deferred are loaded
         immediately when the queryset is evaluated.
         """
+        self._not_support_combined_queries('only')
         if self._fields is not None:
             raise TypeError("Cannot call only() after .values() or .values_list()")
         if fields == (None,):
@@ -1312,6 +1319,13 @@ class QuerySet:
                 )
             )
 
+    def _not_support_combined_queries(self, operation_name):
+        if self.query.combinator:
+            raise NotSupportedError(
+                'Calling QuerySet.%s() after %s() is not supported.'
+                % (operation_name, self.query.combinator)
+            )
+
 
 class InstanceCheckMeta(type):
     def __instancecheck__(self, instance):

tests/queries/test_qs_combinators.py

@@ -1,3 +1,4 @@
+from django.db import connection
 from django.db.models import Exists, F, IntegerField, OuterRef, Value
 from django.db.utils import DatabaseError, NotSupportedError
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
@@ -258,3 +259,29 @@ class QuerySetSetOperationTests(TestCase):
         numbers = list(range(10))
         self.assertNumbersEqual(union.order_by('num'), numbers)
         self.assertNumbersEqual(union.order_by('other_num'), reversed(numbers))
+
+    def test_unsupported_operations_on_combined_qs(self):
+        qs = Number.objects.all()
+        msg = 'Calling QuerySet.%s() after %s() is not supported.'
+        combinators = ['union']
+        if connection.features.supports_select_difference:
+            combinators.append('difference')
+        if connection.features.supports_select_intersection:
+            combinators.append('intersection')
+        for combinator in combinators:
+            for operation in (
+                'annotate',
+                'defer',
+                'exclude',
+                'extra',
+                'filter',
+                'only',
+                'prefetch_related',
+                'select_related',
+            ):
+                with self.subTest(combinator=combinator, operation=operation):
+                    with self.assertRaisesMessage(
+                        NotSupportedError,
+                        msg % (operation, combinator),
+                    ):
+                        getattr(getattr(qs, combinator)(qs), operation)()