fix: issues discovered while using REST API

Can't use only("pk") because of possible select_related clauses in the
Collections queryset.

openedx_learning/apps/authoring/collections/api.py

@@ -101,12 +101,13 @@ def add_to_collections(
     """
     collection_objects = []
     object_ids = contents_qset.values_list("pk", flat=True)
+    collection_ids = collections_qset.values_list("pk", flat=True)
 
-    for collection in collections_qset.only("pk").all():
+    for collection_id in collection_ids:
         for object_id in object_ids:
             collection_objects.append(
                 CollectionObject(
-                    collection_id=collection.pk,
+                    collection_id=collection_id,
                     object_id=object_id,
                 )
             )
@@ -140,16 +141,17 @@ def remove_from_collections(
     """
     total_deleted = 0
     object_ids = contents_qset.values_list("pk", flat=True)
+    collection_ids = collections_qset.values_list("pk", flat=True)
     modified_collection_ids = []
 
-    for collection in collections_qset.only("pk").all():
+    for collection_id in collection_ids:
         num_deleted, _ = CollectionObject.objects.filter(
-            collection_id=collection.pk,
+            collection_id=collection_id,
             object_id__in=object_ids,
         ).delete()
 
         if num_deleted:
-            modified_collection_ids.append(collection.pk)
+            modified_collection_ids.append(collection_id)
 
         total_deleted += num_deleted