support zunionstore in redis (#410)

core/stores/redis/redis.go

@@ -51,7 +51,8 @@ type (
 	Pipeliner = red.Pipeliner
 
 	// Z represents sorted set member.
-	Z = red.Z
+	Z      = red.Z
+	ZStore = red.ZStore
 
 	IntCmd   = red.IntCmd
 	FloatCmd = red.FloatCmd
@@ -1394,6 +1395,20 @@ func (s *Redis) Zrevrank(key string, field string) (val int64, err error) {
 	return
 }
 
+func (s *Redis) Zunionstore(dest string, store ZStore, keys ...string) (val int64, err error) {
+	err = s.brk.DoWithAcceptable(func() error {
+		conn, err := getRedis(s)
+		if err != nil {
+			return err
+		}
+
+		val, err = conn.ZUnionStore(dest, store, keys...).Result()
+		return err
+	}, acceptable)
+
+	return
+}
+
 func (s *Redis) String() string {
 	return s.Addr
 }