Follow ReferenceFields in EmbeddedDocuments with select_related

For the following structure:

    class Playlist(Document):
        items = ListField(EmbeddedDocumentField("PlaylistItem"))

    class PlaylistItem(EmbeddedDocument):
        song = ReferenceField("Song")

    class Song(Document):
        title = StringField()

this patch prevents the N+1 queries otherwise required to fetch all
the `Song` instances referenced by all the `PlaylistItem`s.

docs/changelog.rst

@@ -6,6 +6,7 @@ Changelog
 Changes in 0.9.X - DEV
 ======================
 
+- Follow ReferenceFields in EmbeddedDocuments with select_related #690
 - Added preliminary support for text indexes #680
 - Added `elemMatch` operator as well - `match` is too obscure #653
 - Added support for progressive JPEG #486 #548

mongoengine/dereference.py

@@ -95,7 +95,7 @@ class DeReference(object):
         # Recursively find dbreferences
         depth += 1
         for k, item in iterator:
-            if isinstance(item, Document):
+            if isinstance(item, (Document, EmbeddedDocument)):
                 for field_name, field in item._fields.iteritems():
                     v = item._data.get(field_name, None)
                     if isinstance(v, (DBRef)):
@@ -202,7 +202,7 @@ class DeReference(object):
 
             if k in self.object_map and not is_list:
                 data[k] = self.object_map[k]
-            elif isinstance(v, Document):
+            elif isinstance(v, (Document, EmbeddedDocument)):
                 for field_name, field in v._fields.iteritems():
                     v = data[k]._data.get(field_name, None)
                     if isinstance(v, (DBRef)):

tests/test_dereference.py

@@ -1219,6 +1219,30 @@ class FieldTest(unittest.TestCase):
         page = Page.objects.first()
         self.assertEqual(page.tags[0], page.posts[0].tags[0])
 
+    def test_select_related_follows_embedded_referencefields(self):
+        class Playlist(Document):
+            items = ListField(EmbeddedDocumentField("PlaylistItem"))
+
+        class PlaylistItem(EmbeddedDocument):
+            song = ReferenceField("Song")
+
+        class Song(Document):
+            title = StringField()
+
+        Playlist.drop_collection()
+        Song.drop_collection()
+
+        songs = [Song.objects.create(title="song %d" % i) for i in range(3)]
+        items = [PlaylistItem(song=song) for song in songs]
+        playlist = Playlist.objects.create(items=items)
+
+        with query_counter() as q:
+            self.assertEqual(q, 0)
+
+            playlist = Playlist.objects.first().select_related()
+            songs = [item.song for item in playlist.items]
+
+            self.assertEqual(q, 2)
+
 if __name__ == '__main__':
     unittest.main()
-