I'm learning to use mongodb. I made 3 collections (User, Enrollment, Courses)

```
User
+--------------------------------------+---------+--------+
| _id                                  | user_id | name   |
+--------------------------------------+---------+--------+
| ObjectId("5ee6c0511d0843811413b225") | 1       | John   |
+--------------------------------------+---------+--------+
| ObjectId("5ef9e9b11db598099e183319") | 2       | Bob    |
+--------------------------------------+---------+--------+

Courses
+--------------------------------------+---------+-----------------+
| _id                                  | courseID| courseName      |
+--------------------------------------+---------+-----------------+
| ObjectId("5ef9d1b28e08e1c04ac9530b") | "1111"  | English         |
+--------------------------------------+---------+-----------------+
| ObjectId("5ef9db2bdd883a3444dd396a") | "2222"  | Algebra 1       |
+--------------------------------------+---------+-----------------+
| ObjectId("5ef9ea212c72182edf809a52") | "3333"  | World History   |
+--------------------------------------+---------+-----------------+
| ObjectId("5ee6c0511d0843811413b226") | "4444"  | Algebra 2       |
+--------------------------------------+---------+-----------------+

Enrollment
+---------+----------+
| user_id | courseID |
+---------+----------+
| 1       | "1111"   |
+---------+----------+
| 2       | "2222"   |
+---------+----------+
| 1       | "2222"   |
+---------+----------+
| 1       | "4444"   |
+---------+----------+

```

Everytime user click on one of the courses will added into enrollment table.
I am able to show the enrolled list for user, but I have no idea how to show courses that have not been enrolled by user.

I would like to know how to show courses that have not been added to enrollment table by user?
I tried to use `user_id: { $ne: 1 }`, but doesn't seem show the correct courses that have not been enrolled.

How to show correctly?