mongoose中间件使用时的疑惑
在用到pre(‘findOneAndUpdate’,function(next) {…})时，从回调的this里面获取到的字段是undefined，如果schema定义了字段meta，然后在执行findOneAndUpdate操作后，pre操作中回调获取this.meta是undefined，目前对mongoose还不太熟，不知从何分析
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自问自答一波，引用
Query middleware differs from document middleware in a subtle but important way: in document middleware, this refers to the document being updated. In query middleware, mongoose doesn’t necessarily have a reference to the document being updated, so this refers to the query object rather than the document being updated.
