add yield_per_query() helper

app/models.py

@@ -65,6 +65,11 @@ class ModelMixin(object):
     def query(cls):
         return Session.query(cls)
 
+    @classmethod
+    def yield_per_query(cls, page=1000):
+        """to be used when iterating on a big table to avoid taking all the memory"""
+        return Session.query(cls).yield_per(page).enable_eagerloads(False)
+
     @classmethod
     def get(cls, id):
         return Session.query(cls).get(id)