db={"users":[{"_id":ObjectId("655c3285822c4a52150c30a7"),"name":"kunal","email":"kunal@gmail.com"},{"_id":ObjectId("655c3285822c4a52150c30ab"),"name":"Amit","email":"amit@gmail.com"}],"user_posts":[{"_id":ObjectId("655c3285822c4a52150c30a8"),"user_id":"655c3285822c4a52150c30a7","post_name":"Test Post"},{"_id":ObjectId("655c3285822c4a52150c30a9"),"user_id":"655c3285822c4a52150c30a7","post_name":"Test Post 2"},{"_id":ObjectId("655c3285822c4a52150c30aa"),"user_id":"655c3285822c4a52150c30a7","post_name":"Test Post 3"}]}

db.user_posts.aggregate([{"$lookup":{"as":"user_info","from":"users","foreignField":"_id","localField":"user_id"}}])