add index for collection
add index for collection

skinnyworm/wechat-rails,EndingChaung/wechat,yxf/wechat,Eric-Guo/wechat,Eric-Guo/wechat,wolflee/wechat-rails,zfben/wechat,wolflee/wechat-rails,TangMonk/wechat,JeeLiu/wechat