Cache serialized entities when fetching many annotations

For a scene with ~5600 annotations, of which ~3100 are entities, this
cuts fetching the scene from 12 seconds to 2 seconds.

pandora/annotation/models.py

@@ -208,13 +208,36 @@ class Annotation(models.Model):
         from entity.models import Entity
         return Entity.get(self.value)
 
+    def _get_entity_json(self, user=None, entity_cache=None):
+        """When serializing many annotations pointing to the same entity, it is expensive to
+        repeatedly look up and serialize the same entity.
+
+        TODO: if Entity were a (nullable) foreign key of Annotation, we could just:
+
+            prefetch_related('entity', 'entity__user', 'entity__documents')
+
+        before serializing the annotations, which would make self.entity.json(user=user) cheap and
+        all this unnecessary.
+        """
+        if entity_cache is not None and self.value in entity_cache:
+            return entity_cache[self.value]
+
+        entity = self.get_entity()
+        entity_json = entity.json(user=user)
+        value = entity.annotation_value()
+
+        if entity_cache is not None:
+            entity_cache[self.value] = (entity_json, value)
+
+        return (entity_json, value)
+
     annotation_keys = (
         'id', 'in', 'out', 'value', 'created', 'modified',
         'duration', 'layer', 'item', 'videoRatio', 'languages', 
         'entity', 'event', 'place'
     )
     _clip_keys = ('hue', 'lightness', 'saturation', 'volume')
-    def json(self, layer=False, keys=None, user=None):
+    def json(self, layer=False, keys=None, user=None, entity_cache=None):
         j = {
             'user': self.user.username,
             'id': self.public_id,
@@ -232,9 +255,8 @@ class Annotation(models.Model):
         l = self.get_layer()
         if l['type'] == 'entity':
             try:
-                entity = self.get_entity()
-                j['entity'] = entity.json(user=user)
-                j['value'] = entity.annotation_value()
+                (j['entity'], j['value']) = self._get_entity_json(
+                    user=user, entity_cache=entity_cache)
             except:
                 j['entity'] = {}
         elif l['type'] == 'event':

pandora/annotation/views.py

@@ -100,7 +100,11 @@ def findAnnotations(request, data):
     qs = order_query(query['qs'], query['sort'])
     if 'keys' in data:
         qs = qs.select_related()[query['range'][0]:query['range'][1]]
-        response['data']['items'] = [p.json(keys=data['keys']) for p in qs]
+        entity_cache = {}
+        response['data']['items'] = [
+            p.json(keys=data['keys'], entity_cache=entity_cache)
+            for p in qs
+        ]
     elif 'position' in query:
         ids = [i.public_id for i in qs]
         data['conditions'] = data['conditions'] + {

pandora/clip/models.py

@@ -32,12 +32,14 @@ def get_layers(item, interval=None, user=None):
         start, end = interval
         qs = qs.filter(start__lt=end, end__gt=start)
 
+    entity_cache = {}
+
     for a in qs.order_by('start').select_related('user'):
-        if a.layer in private:
-            if a.user == user:
-                layers[a.layer].append(a.json(user=user))
-        else:
-            layers[a.layer].append(a.json(user=user))
+        if a.layer in private and a.user != user:
+            continue
+
+        layers[a.layer].append(a.json(user=user, entity_cache=entity_cache))
+
     return layers
 
 
@@ -107,7 +109,10 @@ class MetaClip:
                 annotations = self.annotations.all()
                 if qs:
                     annotations = annotations.filter(qs)
-                j['annotations'] = [a.json(keys=['value', 'id', 'layer']) for a in annotations]
+                entity_cache = {}
+                j['annotations'] = [
+                    a.json(keys=['value', 'id', 'layer'], entity_cache=entity_cache) for a in annotations
+                ]
             if 'layers' in keys:
                 j['layers'] = self.get_layers()
             if 'cuts' in keys: