mongodb如何判断字段是否存在

db.users.find({age: {$exists: true}});

db.users.find({name: {$exists: false}});

> db.c1.find();

{ "_id" : ObjectId("4fb4a773afa87dc1bed9432d"), "age" : 20, "length" : 30 }

{ "_id" : ObjectId("4fb4a7e1afa87dc1bed9432e"), "age_1" : 20, "length_1" : 30 }

> db.c1.find({age:{$exists:true}});

{ "_id" : ObjectId("4fb4a773afa87dc1bed9432d"), "age" : 20, "length" : 30 }

db.users.find({sex: {$exists: true}});