Use a DataLoader to efficiently batch load homeworlds

See graphql-python#12 for background.

starwars/schema.py

@@ -10,6 +10,16 @@
 
 import models
 
+from promise import Promise
+from promise.dataloader import DataLoader
+
+
+class PlanetLoader(DataLoader):
+    def batch_load_fn(self, keys):
+        return Promise.resolve(models.Planet.objects.filter(pk__in=keys))
+
+planet_loader = PlanetLoader()
+
 
 def connection_for_type(_type):
     class Connection(graphene.Connection):
@@ -94,6 +104,9 @@ def resolve_hair_colors(self):
     def resolve_skin_colors(self):
         return [c.strip() for c in self.skin_colors.split(',')]
 
+    def resolve_homeworld(self, *args):
+        return planet_loader.load(self.id)
+
     class Meta:
         model = models.Species
         interfaces = (Node, )