when removing field from _field_data_cache when rebinding a module to a new…

when removing field from _field_data_cache when rebinding a module to a new user, also remove it from _dirty_fields (TNL-2640)

common/lib/xmodule/xmodule/x_module.py

@@ -587,6 +587,12 @@ class XModuleMixin(XModuleFields, XBlock):
 
             if field.scope.user == UserScope.ONE:
                 field._del_cached_value(self)  # pylint: disable=protected-access
+                # not the most elegant way of doing this, but if we're removing
+                # a field from the module's field_data_cache, we should also
+                # remove it from its _dirty_fields
+                # pylint: disable=protected-access
+                if field in self._dirty_fields:
+                    del self._dirty_fields[field]
 
         # Set the new xmodule_runtime and field_data (which are user-specific)
         self.xmodule_runtime = xmodule_runtime

lms/djangoapps/courseware/tests/test_module_render.py

@@ -1360,23 +1360,44 @@ class TestRebindModule(TestSubmittingProblems):
         super(TestRebindModule, self).setUp()
         self.homework = self.add_graded_section_to_course('homework')
         self.lti = ItemFactory.create(category='lti', parent=self.homework)
+        self.problem = ItemFactory.create(category='problem', parent=self.homework)
         self.user = UserFactory.create()
         self.anon_user = AnonymousUser()
 
-    def get_module_for_user(self, user):
+    def get_module_for_user(self, user, item=None):
         """Helper function to get useful module at self.location in self.course_id for user"""
         mock_request = MagicMock()
         mock_request.user = user
         field_data_cache = FieldDataCache.cache_for_descriptor_descendents(
             self.course.id, user, self.course, depth=2)
 
+        if item is None:
+            item = self.lti
+
         return render.get_module(  # pylint: disable=protected-access
             user,
             mock_request,
-            self.lti.location,
+            item.location,
             field_data_cache,
         )._xmodule
 
+    def test_rebind_module_to_new_users(self):
+        module = self.get_module_for_user(self.user, self.problem)
+
+        # Bind the module to another student, which will remove "correct_map"
+        # from the module's _field_data_cache and _dirty_fields.
+        user2 = UserFactory.create()
+        module.descriptor.bind_for_student(module.system, user2.id)
+
+        # XBlock's save method assumes that if a field is in _dirty_fields,
+        # then it's also in _field_data_cache. If this assumption
+        # doesn't hold, then we get an error trying to bind this module
+        # to a third student, since we've removed "correct_map" from
+        # _field_data cache, but not _dirty_fields, when we bound
+        # this module to the second student. (TNL-2640)
+        user3 = UserFactory.create()
+        module.descriptor.bind_for_student(module.system, user3.id)
+
     def test_rebind_noauth_module_to_user_not_anonymous(self):
         """
         Tests that an exception is thrown when rebind_noauth_module_to_user is run from a