Remove the ability to `select_for_updates` from FieldDataCache.

The only consumer of that functionality (the XQueue callback) already
does retries, so newly introduce integrity errors (due to multiple
commiters trying to update a StudentModule) won't break the XQueue
processing pipeline.

lms/djangoapps/courseware/model_data.py

@@ -44,18 +44,7 @@ def chunks(items, chunk_size):
     return (items[i:i + chunk_size] for i in xrange(0, len(items), chunk_size))
 
 
-def _query(model_class, select_for_update, **kwargs):
-    """
-    Queries model_class with **kwargs, optionally adding select_for_update if
-    `select_for_update` is True.
-    """
-    query = model_class.objects
-    if select_for_update:
-        query = query.select_for_update()
-    query = query.filter(**kwargs)
-    return query
-
-def _chunked_query(model_class, select_for_update, chunk_field, items, chunk_size=500, **kwargs):
+def _chunked_query(model_class, chunk_field, items, chunk_size=500, **kwargs):
     """
     Queries model_class with `chunk_field` set to chunks of size `chunk_size`,
     and all other parameters from `**kwargs`.
@@ -64,7 +53,7 @@ def _chunked_query(model_class, select_for_update, chunk_field, items, chunk_siz
     that can be put into a single query.
     """
     res = chain.from_iterable(
-        _query(model_class, select_for_update, **dict([(chunk_field, chunk)] + kwargs.items()))
+        model_class.objects.filter(**dict([(chunk_field, chunk)] + kwargs.items()))
         for chunk in chunks(items, chunk_size)
     )
     return res
@@ -358,11 +347,10 @@ class UserStateCache(object):
     """
     Cache for Scope.user_state xblock field data.
     """
-    def __init__(self, user, course_id, select_for_update=False):
+    def __init__(self, user, course_id):
         self._cache = {}
         self.course_id = course_id
         self.user = user
-        self.select_for_update = select_for_update
 
     def cache_fields(self, fields, xblocks, aside_types):  # pylint: disable=unused-argument
         """
@@ -376,7 +364,6 @@ class UserStateCache(object):
         """
         query = _chunked_query(
             StudentModule,
-            self.select_for_update,
             'module_state_key__in',
             _all_usage_keys(xblocks, aside_types),
             course_id=self.course_id,
@@ -549,10 +536,9 @@ class UserStateSummaryCache(DjangoOrmFieldCache):
     """
     Cache for Scope.user_state_summary xblock field data.
     """
-    def __init__(self, course_id, select_for_update=False):
+    def __init__(self, course_id):
         super(UserStateSummaryCache, self).__init__()
         self.course_id = course_id
-        self.select_for_update = select_for_update
 
     def _create_object(self, kvs_key):
         """
@@ -583,7 +569,6 @@ class UserStateSummaryCache(DjangoOrmFieldCache):
         """
         return _chunked_query(
             XModuleUserStateSummaryField,
-            self.select_for_update,
             'usage_id__in',
             _all_usage_keys(xblocks, aside_types),
             field_name__in=set(field.name for field in fields),
@@ -612,10 +597,9 @@ class PreferencesCache(DjangoOrmFieldCache):
     """
     Cache for Scope.preferences xblock field data.
     """
-    def __init__(self, user, select_for_update=False):
+    def __init__(self, user):
         super(PreferencesCache, self).__init__()
         self.user = user
-        self.select_for_update = select_for_update
 
     def _create_object(self, kvs_key):
         """
@@ -647,7 +631,6 @@ class PreferencesCache(DjangoOrmFieldCache):
         """
         return _chunked_query(
             XModuleStudentPrefsField,
-            self.select_for_update,
             'module_type__in',
             _all_block_types(xblocks, aside_types),
             student=self.user.pk,
@@ -677,10 +660,9 @@ class UserInfoCache(DjangoOrmFieldCache):
     """
     Cache for Scope.user_info xblock field data
     """
-    def __init__(self, user, select_for_update=False):
+    def __init__(self, user):
         super(UserInfoCache, self).__init__()
         self.user = user
-        self.select_for_update = select_for_update
 
     def _create_object(self, kvs_key):
         """
@@ -709,9 +691,7 @@ class UserInfoCache(DjangoOrmFieldCache):
             aside_types (list of str): Asides to load field for (which annotate the supplied
                 xblocks).
         """
-        return _query(
-            XModuleStudentInfoField,
-            self.select_for_update,
+        return XModuleStudentInfoField.objects.filter(
             student=self.user.pk,
             field_name__in=set(field.name for field in fields),
         )
@@ -751,11 +731,9 @@ class FieldDataCache(object):
         descriptors: A list of XModuleDescriptors.
         course_id: The id of the current course
         user: The user for which to cache data
-        select_for_update: True if rows should be locked until end of transaction
+        select_for_update: Ignored
         asides: The list of aside types to load, or None to prefetch no asides.
         '''
-        self.select_for_update = select_for_update
-
         if asides is None:
             self.asides = []
         else:
@@ -769,19 +747,15 @@ class FieldDataCache(object):
             Scope.user_state: UserStateCache(
                 self.user,
                 self.course_id,
-                self.select_for_update,
             ),
             Scope.user_info: UserInfoCache(
                 self.user,
-                self.select_for_update,
             ),
             Scope.preferences: PreferencesCache(
                 self.user,
-                self.select_for_update,
             ),
             Scope.user_state_summary: UserStateSummaryCache(
                 self.course_id,
-                self.select_for_update,
             ),
         }
         self.add_descriptors_to_cache(descriptors)
@@ -849,7 +823,7 @@ class FieldDataCache(object):
             the supplied descriptor. If depth is None, load all descendent StudentModules
         descriptor_filter is a function that accepts a descriptor and return wether the StudentModule
             should be cached
-        select_for_update: Flag indicating whether the rows should be locked until end of transaction
+        select_for_update: Ignored
         """
         cache = FieldDataCache([], course_id, user, select_for_update, asides=asides)
         cache.add_descriptor_descendents(descriptor, depth, descriptor_filter)

lms/djangoapps/courseware/module_render.py

@@ -737,7 +737,6 @@ def load_single_xblock(request, user_id, course_id, usage_key_string):
         user,
         modulestore().get_item(usage_key),
         depth=0,
-        select_for_update=True
     )
     instance = get_module(user, request, usage_key, field_data_cache, grade_bucket_type='xqueue')
     if instance is None: