Bulk-reads and Request caching in Course Grade Report

openedx/core/djangoapps/user_api/course_tag/api.py

@@ -7,6 +7,8 @@ Stores global metadata using the UserPreference model, and per-course metadata u
 UserCourseTag model.
 """
 
+from collections import defaultdict
+from request_cache import get_cache
 from ..models import UserCourseTag
 
 # Scopes
@@ -15,6 +17,42 @@ from ..models import UserCourseTag
 COURSE_SCOPE = 'course'
 
 
+class BulkCourseTags(object):
+    CACHE_NAMESPACE = u'user_api.course_tag.api'
+
+    @classmethod
+    def prefetch(cls, course_id, users):
+        """
+        Prefetches the value of the course tags for the specified users
+        for the specified course_id.
+
+        Args:
+            users: iterator of User objects
+            course_id: course identifier (CourseKey)
+
+        Returns:
+            course_tags: a dict of dicts,
+                where the primary key is the user's id
+                and the secondary key is the course tag's key
+        """
+        course_tags = defaultdict(dict)
+        for tag in UserCourseTag.objects.filter(user__in=users, course_id=course_id).select_related('user__id'):
+            course_tags[tag.user.id][tag.key] = tag.value
+        get_cache(cls.CACHE_NAMESPACE)[cls._cache_key(course_id)] = course_tags
+
+    @classmethod
+    def get_course_tag(cls, user_id, course_id, key):
+        return get_cache(cls.CACHE_NAMESPACE)[cls._cache_key(course_id)][user_id][key]
+
+    @classmethod
+    def is_prefetched(cls, course_id):
+        return cls._cache_key(course_id) in get_cache(cls.CACHE_NAMESPACE)
+
+    @classmethod
+    def _cache_key(cls, course_id):
+        return u'course_tag.{}'.format(course_id)
+
+
 def get_course_tag(user, course_id, key):
     """
     Gets the value of the user's course tag for the specified key in the specified
@@ -28,6 +66,11 @@ def get_course_tag(user, course_id, key):
     Returns:
         string value, or None if there is no value saved
     """
+    if BulkCourseTags.is_prefetched(course_id):
+        try:
+            return BulkCourseTags.get_course_tag(user.id, course_id, key)
+        except KeyError:
+            return None
     try:
         record = UserCourseTag.objects.get(
             user=user,

openedx/core/djangoapps/user_api/partition_schemes.py

@@ -70,7 +70,7 @@ class RandomUserPartitionScheme(object):
                     exc_info=True
                 )
 
-        if group is None and assign:
+        if group is None and assign and not course_tag_api.BulkCourseTags.is_prefetched(course_key):
             if not user_partition.groups:
                 raise UserPartitionError('Cannot assign user to an empty user partition')
 

openedx/core/djangoapps/user_api/tests/test_partition_schemes.py

@@ -26,6 +26,11 @@ class MemoryCourseTagAPI(object):
         """Gets the value of ``key``"""
         self._tags[course_id][key] = value
 
+    class BulkCourseTags(object):
+        @classmethod
+        def is_prefetched(self, course_id):
+            return False
+
 
 class TestRandomUserPartitionScheme(PartitionTestCase):
     """