Add missing Count api

impl/memory/datastore_query_execution.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 memory
 
 import (
         "bytes"
         "fmt"
 
@@ skipping 145 matching lines @@
                 raw[i] = suffix[:offset]
                 suffix = suffix[offset:]
                 if needInvert {
                         raw[i] = invert(raw[i])
                 }
         }
 
         return
 }
 
+func countQuery(fq *ds.FinalizedQuery, ns string, isTxn bool, idx, head *memStore) (ret int64, err error) {
+        if len(fq.Project()) == 0 && !fq.KeysOnly() {
+                fq, err = fq.Original().KeysOnly(true).Finalize()
+                if err != nil {
+                        return
+                }
+        }
+        err = executeQuery(fq, ns, isTxn, idx, head, func(_ *ds.Key, _ ds.PropertyMap, _ ds.CursorCB) bool {
+                ret++
+                return true
+        })
+        return
+}
+
 func executeQuery(fq *ds.FinalizedQuery, ns string, isTxn bool, idx, head *memStore, cb ds.RawRunCB) error {
         rq, err := reduce(fq, ns, isTxn)
         if err == ds.ErrNullQuery {
                 return nil
         }
         if err != nil {
                 return err
         }
 
         idxs, err := getIndexes(rq, idx)
@@ skipping 53 matching lines @@
                         impossible(fmt.Errorf("decoded index row doesn't end with a Key: %#v", keyProp))
                 }
 
                 return strategy.handle(
                         rawData, decodedProps, keyProp.Value().(*ds.Key),
                         getCursorFn(suffix))
         })
 
         return nil
 }