mongoose报错：DeprecationWarning: collection.ensureIndex is deprecated. Use createIndexes instead

mongoose报错：
(node:9716)DeprecationWarning: collection.ensureIndex is deprecated. Use createIndexes instead
解决方法：

//链接数据库
mongoose.set('useCreateIndex', true) //加上这个
mongoose.connect(db, { useNewUrlParser: true })