db={"users":[{_id:"1",name:"testsuser1",family:"123"},{_id:"2",name:"testsuser2",family:"678"},{_id:"3",name:"testsuser3",family:"123"},{_id:"4",name:"testsuser4",family:"897"}],"posts":[{_id:"5f60c3b7f93d8e00a1cdf414",postTitle:"ABC",user:"1"},{_id:"5f60c3b7f93d8e00a1cdf415",postTitle:"XYZ",user:"2"},{_id:"5f60c3b7f93d8e00a1cdf416",postTitle:"KLM",user:"1"},{_id:"5f60c3b7f93d8e00a1cdf417",postTitle:"DFG",user:"3"}]}

db.users.aggregate([{"$match":{"family":"123"}},{"$lookup":{"from":"posts","localField":"_id","foreignField":"user","as":"posts"}}])