Fixed #22023 -- Raised an error for values() followed by defer() or only().

Previously, doing so resulted in invalid data or crash.

Thanks jtiai for the report and Karol Jochelson,
Jakub Nowak, Loic Bistuer, and Baptiste Mispelon for reviews.

django/db/models/query.py

@@ -1060,6 +1060,12 @@ class ValuesQuerySet(QuerySet):
         # QuerySet.clone() will also set up the _fields attribute with the
         # names of the model fields to select.
 
+    def only(self, *fields):
+        raise NotImplementedError("ValuesQuerySet does not implement only()")
+
+    def defer(self, *fields):
+        raise NotImplementedError("ValuesQuerySet does not implement defer()")
+
     def iterator(self):
         # Purge any extra columns that haven't been explicitly asked for
         extra_names = list(self.query.extra_select)

docs/ref/models/querysets.txt

@@ -502,14 +502,23 @@ A few subtleties that are worth mentioning:
   made after a ``values()`` call will have its extra selected fields
   ignored.
 
+* Calling :meth:`only()` and :meth:`defer()` after ``values()`` doesn't make
+  sense, so doing so will raise a ``NotImplementedError``.
+
+.. versionadded:: 1.7
+
+    The last point above is new. Previously, calling :meth:`only()` and
+    :meth:`defer()` after ``values()`` was allowed, but it either crashed or
+    returned incorrect results.
+
 A ``ValuesQuerySet`` is useful when you know you're only going to need values
 from a small number of the available fields and you won't need the
 functionality of a model instance object. It's more efficient to select only
 the fields you need to use.
 
-Finally, note a ``ValuesQuerySet`` is a subclass of ``QuerySet``, so it has all
-methods of ``QuerySet``. You can call ``filter()`` on it, or ``order_by()``, or
-whatever. Yes, that means these two calls are identical::
+Finally, note that a ``ValuesQuerySet`` is a subclass of ``QuerySet`` and it
+implements most of the same methods. You can call ``filter()`` on it,
+``order_by()``, etc. That means that these two calls are identical::
 
     Blog.objects.values().order_by('id')
     Blog.objects.order_by('id').values()

docs/releases/1.7.txt

@@ -616,6 +616,12 @@ Models
   :attr:`~django.db.models.ForeignKey.limit_choices_to` when defining a
   ``ForeignKey`` or ``ManyToManyField``.
 
+* Calling :meth:`only() <django.db.models.query.QuerySet.only>` and
+  :meth:`defer() <django.db.models.query.QuerySet.defer>` on the result of
+  :meth:`QuerySet.values() <django.db.models.query.QuerySet.values>` now raises
+  an error (before that, it would either result in a database error or
+  incorrect data).
+
 Signals
 ^^^^^^^
 

tests/queries/tests.py

@@ -1275,6 +1275,16 @@ class Queries3Tests(BaseQuerysetTest):
             Item.objects.datetimes, 'name', 'month'
         )
 
+    def test_ticket22023(self):
+        # only() and defer() are not applicable for ValuesQuerySet
+        with self.assertRaisesMessage(NotImplementedError,
+                "ValuesQuerySet does not implement only()"):
+            Valid.objects.values().only()
+
+        with self.assertRaisesMessage(NotImplementedError,
+                "ValuesQuerySet does not implement defer()"):
+            Valid.objects.values().defer()
+
 
 class Queries4Tests(BaseQuerysetTest):
     def setUp(self):