db={"users":[{"_id":{"$oid":"610bcc467b0c4008346547b8"},"name":"xyz","email":"xyz@gmail.com","password":"xyz","gender":"MALE"}],"posts":[{"_id":{"$oid":"610bce417b0c4008346547bc"},"image":"myImage 1","caption":"r/Mars","user_id":"610bcc467b0c4008346547b8"},{"_id":{"$oid":"610bce417b0c4008346547be"},"image":"myImage 2","caption":"hmm","user_id":"610bcc467b0c4008346547b8"},{"_id":{"$oid":"610bce417b0c4008346547bd"},"image":"myImage 3","caption":"..","user_id":"610bcc467b0c4008346547b8"}]}

db.users.aggregate([{"$addFields":{"userStrId":{"$toString":"$_id"}}},{"$lookup":{"from":"posts","localField":"userStrId","foreignField":"user_id","as":"user_posts"}},{$addFields:{user_posts:{$map:{input:"$user_posts",in:{$mergeObjects:["$$this",{user_id:"$userStrId",post_id:{$toString:"$$this._id"}}]}}}}}])