db={"users":[{"_id":ObjectId("5a934e000102030405000002"),"first_name":"bob","last_name":"smith","user_email":"bobsmith@gmail.com"},{"_id":ObjectId("5a934e000102030405000003"),"first_name":"sally","last_name":"jones","user_email":"sallyjones@gmail.com"}],"coupons":[{"_id":ObjectId("5a934e000102030405000004"),"user":1,"email":"bobsmith@gmail.com","couponCode":"abc123"},{"_id":ObjectId("5a934e000102030405000005"),"user":2,"email":"sallyjones@gmail.com","couponCode":"xyz789"}]}

db.coupons.aggregate([{"$lookup":{"from":"users","localField":"email","foreignField":"user_email","as":"user_details"}},{"$unwind":"$user_details"}])