Add missing Count api

service/datastore/raw_interface.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package datastore
 
 import (
         "fmt"
 
         "golang.org/x/net/context"
@@ skipping 111 matching lines @@
         // be decoded by the implementation.
         DecodeCursor(s string) (Cursor, error)
 
         // Run executes the given query, and calls `cb` for each successfully item.
         //
         // NOTE: Implementations and filters are guaranteed that:
         //   - query is not nil
         //   - cb is not nil
         Run(q *FinalizedQuery, cb RawRunCB) error
 
+        // Count executes the given query and returns the number of entries which
+        // match it.
+        Count(q *FinalizedQuery) (int64, error)
+
         // GetMulti retrieves items from the datastore.
         //
         // Callback execues once per key, in the order of keys. Callback may not
         // execute at all if there's a server error. If callback is nil, this
         // method does nothing.
         //
         // meta is used to propagate metadata from higher levels.
         //
         // NOTE: Implementations and filters are guaranteed that:
         //   - len(keys) > 0
@@ skipping 22 matching lines @@
         //   - len(keys) > 0
         //   - all keys are Valid, !Incomplete, and in the current namespace
         //   - none keys of the keys are 'special' (use a kind prefixed with '__')
         //   - cb is not nil
         DeleteMulti(keys []*Key, cb DeleteMultiCB) error
 
         // Testable returns the Testable interface for the implementation, or nil if
         // there is none.
         Testable() Testable
 }