Mongo查询$ exists：false行为异常

Question

{ "_id" : ObjectId("5c541529fc3e371af7720346"), "name" : "Sanjeev", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : 122311223 }
{ "_id" : ObjectId("5c541529fc3e371af7720347"), "name" : "siddhanth", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : "122311223", "age" : 30 }
{ "_id" : ObjectId("5c54158efc3e371af7720348"), "name" : "Sanjeev", "hobbies" : [ { "title" : "Trekking", "skills" : [ "mountainclimbing" ], "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : 122311223 }
{ "_id" : ObjectId("5c54158efc3e371af7720349"), "name" : "siddhanth", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : "122311223", "age" : 30 }

以上是我收藏的一个例子。

当我执行以下查询时

> db.player.find({hobbies:{$elemMatch:{skills:{$exists:true}}}})

我得到以下结果，其中“爱好”具有“技能”

{ "_id" : ObjectId("5c54158efc3e371af7720348"), "name" : "Sanjeev", "hobbies" : [ { "title" : "Trekking", "skills" : [ "mountainclimbing" ], "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : 122311223 }

但是当我执行

> db.player.find({hobbies:{$elemMatch:{'skills':{$exists:false}}}})

我也获得了具有“技能”的记录。

{ "_id" : ObjectId("5c541529fc3e371af7720346"), "name" : "Sanjeev", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : 122311223 }
{ "_id" : ObjectId("5c541529fc3e371af7720347"), "name" : "siddhanth", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : "122311223", "age" : 30 }
{ "_id" : ObjectId("5c54158efc3e371af7720348"), "name" : "Sanjeev", "hobbies" : [ { "title" : "Trekking", "skills" : [ "mountainclimbing" ], "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : 122311223 }
{ "_id" : ObjectId("5c54158efc3e371af7720349"), "name" : "siddhanth", "hobbies" : [ { "title" : "Trekking", "frequency" : 3 }, { "title" : "Cooking", "frequency" : 4 } ], "phone" : "122311223", "age" : 30 }

我是mongodb的新手。我在这里想念什么？你能解释一下吗？

Answer 1

db.player.find({ hobbies:{ $elemMatch:{ skills:{ $exists:true }}}})

$elemMatch遍历数组中的每个元素，并检查条件，如果任何数组元素与返回文档的条件相匹配。

db.player.find({ "hobbies.skills": { $exists: true }})

.dot表示数组中的每个元素是否满足条件，然后才返回文档。