Added ability to have scalar return values instead of partially-populated Document instances.

mongoengine/queryset.py

@@ -411,6 +411,7 @@ class QuerySet(object):
         self._timeout = True
         self._class_check = True
         self._slave_okay = False
+        self._scalar = []
 
         # If inheritance is allowed, only return instances and instances of
         # subclasses of the class being used
@@ -977,6 +978,11 @@ class QuerySet(object):
 
         docs = self._collection.find({'_id': {'$in': object_ids}},
                                      **self._cursor_args)
+        if self._scalar:
+            for doc in docs:
+                doc_map[doc['_id']] = self._get_scalar(
+                        self._document._from_son(doc))
+        else:
             for doc in docs:
                 doc_map[doc['_id']] = self._document._from_son(doc)
 
@@ -988,6 +994,9 @@ class QuerySet(object):
         try:
             if self._limit == 0:
                 raise StopIteration
+            if self._scalar:
+                return self._get_scalar(self._document._from_son(
+                        self._cursor.next()))
             return self._document._from_son(self._cursor.next())
         except StopIteration, e:
             self.rewind()
@@ -1164,6 +1173,9 @@ class QuerySet(object):
             return self
         # Integer index provided
         elif isinstance(key, int):
+            if self._scalar:
+                return self._get_scalar(self._document._from_son(
+                        self._cursor[key]))
             return self._document._from_son(self._cursor[key])
         raise AttributeError
 
@@ -1490,6 +1502,38 @@ class QuerySet(object):
         self.rewind()
         return self
 
+    def _get_scalar(self, doc):
+        def lookup(obj, name):
+            chunks = name.split('__')
+            for chunk in chunks:
+                obj = getattr(obj, chunk)
+            return obj
+        
+        data = [lookup(doc, n) for n in self._scalar]
+        
+        if len(data) == 1:
+            return data[0]
+        
+        return tuple(data)
+
+    def scalar(self, *fields):
+        """Instead of returning Document instances, return either a specific
+        value or a tuple of values in order.
+        
+        This effects all results and can be unset by calling ``scalar``
+        without arguments. Calls ``only`` automatically.
+        
+        :param fields: One or more fields to return instead of a Document.
+        """
+        self._scalar = list(fields)
+        
+        if fields:
+            self.only(*fields)
+        else:
+            self.all_fields()
+        
+        return self
+
     def _sub_js_fields(self, code):
         """When fields are specified with [~fieldname] syntax, where
         *fieldname* is the Python name of a field, *fieldname* will be