db={"gateway":[{"_id":"18001887","mac_id":"18001887","group_id":"0"},{"_id":"18001888","mac_id":"18001888","group_id":"1"},{"_id":"18001889","mac_id":"18001889","group_id":"0"}],"commands":[{"_id":ObjectId("615581dcb9ebca6c37eb39e4"),"org_id":0,"mac_id":"18001887","config":{"user_info":[{"user_id":1,"user_pwd":"123456","mapped_id":1},{"user_id":2,"user_pwd":"123123","mapped_id":3}]}},{"_id":ObjectId("615581dcb9ebca6c37eb39e5"),"org_id":0,"mac_id":"18001889","config":{"slave_id":1}}]}

db.gateway.aggregate([{$match:{group_id:"0"}},{$lookup:{from:"commands",let:{mac_id:"$mac_id"},pipeline:[{$match:{$expr:{$eq:["$mac_id","$$mac_id"]},"config.user_info.mapped_id":1}}],as:"childs"}},{$project:{_id:0,mac_id:1,childs:1}}])