MongoDB - not in other table (many-to-many relationship)

Question

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?

Answer 1

You can use aggregation

Assuming you know user_id , let's say we choose Bob with user_id: 2

You can either start with getting all courses from Course and filter only courses that the user has not enrolled

db.Course.aggregate({
  $lookup: { // "join" with Enrollment
    from: "Enrollment",
    localField: "courseID",
    foreignField: "courseID",
    as: "enrollments"
  },
},
{
  $match: { // only match courses that user_id 2 doesn't exist in list of enrolments
    "enrollments.user_id": {
      $ne: 2
    }
  }
})

Playground

Or start with Enrollment from a particular User and find the courses from Course

db.Enrollment.aggregate({
  $match: { // find enrolments of user_id 2
    user_id: 2
  }
},
{
  $group: { // combine all courseID into an array
    _id: "$user_id",
    courseIDs: {
      $push: "$courseID"
    }
  }
},
{
  $lookup: { // "join" with Course that is not in the list of enrolments
    from: "Course",
    as: "notEnrolledCourses",
    let: {
      courseIDs: "$courseIDs"
    },
    pipeline: [
      {
        $match: {
          $expr: {
            $not: {
              $in: [
                "$courseID",
                "$$courseIDs"
              ]
            }
          }
        }
      }
    ]
  }
})

Playground

Note that above approaches don't have the same output structure, you will have to manipulate the to get to your desired output shape.

Answer 2

With this collection structure:

Step 1 : Fetch user's enrolled courses list. For eg: user 2's courses: ['2222']

Step 2 : Use find query ( { courseID: { $nin: ['2222'] } } ) on Courses collection to fetch un-enrolled courses list.

However, I'd suggest you to update collection's schema by either of following ways:

• Save enrolled courses Ids in User collection in an Array.
• Save Courses in an Array in Enrollment collection.

This way, you can eliminate some processing in Step 1.

Hope this helps!