Optimisation in prefetch_related_objects

django/db/models/query.py

@@ -1545,8 +1545,18 @@ def prefetch_related_objects(result_cache, related_lookups):
             if len(obj_list) == 0:
                 break
 
+            current_lookup = LOOKUP_SEP.join(attrs[0:level+1])
+            if current_lookup in done_queries:
+                # Skip any prefetching, and any object preparation
+                obj_list = done_queries[current_lookup]
+                continue
+
+            # Prepare objects:
             good_objects = True
             for obj in obj_list:
+                # Since prefetching can re-use instances, it is possible to have
+                # the same instance multiple times in obj_list, so obj might
+                # already be prepared.
                 if not hasattr(obj, '_prefetched_objects_cache'):
                     try:
                         obj._prefetched_objects_cache = {}
@@ -1557,14 +1567,6 @@ def prefetch_related_objects(result_cache, related_lookups):
                         # now.
                         good_objects = False
                         break
-                else:
-                    # Since prefetching can re-use instances, it is possible to
-                    # have the same instance multiple times in obj_list. So we
-                    # can reach this branch either because we did all of
-                    # obj_list already, or because we did 'obj' earlier in this
-                    # iteration over obj_list. In the first case we could
-                    # shortcut and exit the loop, but not in the second.
-                    continue
             if not good_objects:
                 break
 
@@ -1589,23 +1591,18 @@ def prefetch_related_objects(result_cache, related_lookups):
                                  "prefetch_related()." % lookup)
 
             if prefetcher is not None and not is_fetched:
-                # Check we didn't do this already
-                current_lookup = LOOKUP_SEP.join(attrs[0:level+1])
-                if current_lookup in done_queries:
-                    obj_list = done_queries[current_lookup]
-                else:
-                    obj_list, additional_prl = prefetch_one_level(obj_list, prefetcher, attr)
-                    # We need to ensure we don't keep adding lookups from the
-                    # same relationships to stop infinite recursion. So, if we
-                    # are already on an automatically added lookup, don't add
-                    # the new lookups from relationships we've seen already.
-                    if not (lookup in auto_lookups and
-                            descriptor in followed_descriptors):
-                        for f in additional_prl:
-                            new_prl = LOOKUP_SEP.join([current_lookup, f])
-                            auto_lookups.append(new_prl)
-                        done_queries[current_lookup] = obj_list
-                    followed_descriptors.add(descriptor)
+                obj_list, additional_prl = prefetch_one_level(obj_list, prefetcher, attr)
+                # We need to ensure we don't keep adding lookups from the
+                # same relationships to stop infinite recursion. So, if we
+                # are already on an automatically added lookup, don't add
+                # the new lookups from relationships we've seen already.
+                if not (lookup in auto_lookups and
+                        descriptor in followed_descriptors):
+                    for f in additional_prl:
+                        new_prl = LOOKUP_SEP.join([current_lookup, f])
+                        auto_lookups.append(new_prl)
+                    done_queries[current_lookup] = obj_list
+                followed_descriptors.add(descriptor)
             else:
                 # Either a singly related object that has already been fetched
                 # (e.g. via select_related), or hopefully some other property