groundwork for deferred fields

mongoengine/base.py

@@ -344,8 +344,12 @@ class BaseDocument(object):
                 return None
             cls = subclasses[class_name]
 
+        present_fields = data.keys()
+
         for field_name, field in cls._fields.items():
             if field.name in data:
                 data[field_name] = field.to_python(data[field.name])
 
-        return cls(**data)
+        obj = cls(**data)
+        obj._present_fields = present_fields
+        return obj

mongoengine/queryset.py

@@ -112,6 +112,7 @@ class QuerySet(object):
         self._accessed_collection = False
         self._query = {}
         self._where_clauses = []
+        self._field_subset = []
 
         # If inheritance is allowed, only return instances and instances of
         # subclasses of the class being used
@@ -188,7 +189,12 @@ class QuerySet(object):
     @property
     def _cursor(self):
         if not self._cursor_obj:
-            self._cursor_obj = self._collection.find(self._query)
+            query_kwargs = {}
+            if self._field_subset:
+                # load only a subset of fields
+                query_kwargs['fields'] = self._field_subset
+            self._cursor_obj = self._collection.find(self._query, **query_kwargs)
+
             # Apply where clauses to cursor
             for js in self._where_clauses:
                 self._cursor_obj.where(js)
@@ -334,6 +340,14 @@ class QuerySet(object):
         elif isinstance(key, int):
             return self._document._from_son(self._cursor[key])
 
+    def only(self, *fields):
+        """Allow only a subset of fields to be loaded. Invalid fields
+        are silently ignored.
+        """
+        fields = list(fields)
+        self._field_subset = fields
+        return self
+
     def order_by(self, *keys):
         """Order the :class:`~mongoengine.queryset.QuerySet` by the keys. The
         order may be specified by prepending each of the keys by a + or a -.