比如：有如下文档: {a:2,b:3}
如何在查询条件中引用key b的值呢？比如我想做：
find({a:{$gt:b＋1}})，但是这样，会提示b没有定义，当然这个例子可以用$where去实现。
但是我只是想知道如何引用b的值，谢谢！
5 回复
貌似这种情况只能用where，摘自官网doc：
Use the $where operator to pass a string containing a JavaScript expression to the query system to provide greater flexibility with queries. Consider the following:
db.collection.find( { $where: "this.a == this.b" } );
$where evaluates JavaScript and cannot take advantage of indexes. Therefore, query performance improves when you express your query using the standard MongoDB operators (e.g., $gt, $in).
In general, you should use $where only when you can’t express your query using another operator. If you must use $where, try to include at least one other standard query operator to filter the result set. Using $where alone requires a table scan.
