How can I populate array of objectIds in mongodb aggregate?

Question

Facing a problem.

I have schema for tags and schema for course. Courses have an array of tags schema's ObjectIds.

How can I populate those tags with full schemas details.

Sample Tag Object
{
   _id: 44ffbvb...,
   title: "Tag 1
}

Sample Course Object

_id: 44ffaab23231,
tags: [ObjectId(44ffbvb...),..]

How can I populate those tags field performing $lookup stage in aggregation when using pipeline?

Answer 1

// Tag: collection name `tags`
[
  {
    _id: ObjectId("44ffbvb..."),
    title: "Tag 1"
  }
]

// Course: collection name `courses`
[
  {
    _id: ObjectId("44ffaab23231"),
    tags: [
      ObjectId("44ffbvb..."),
      ObjectId("44ffbvc..."),
      ObjectId("44ffbvd...")
    ]
  }
]

// Query using aggregate pipeline
db.courses.aggregate([
  {
    $match: {
      "name": "MongoDB"
    }
  },
  {
    $lookup: {
      from: "tags",
      let: { tags: "$tags" },
      pipeline: [
        {
          $match: {
            $expr: { $in: ["$_id", "$$tags"] }
          }
        },
        {
          $project: {
            "title": 1
          }
        }
      ],
      as: "tags"
    }
  }
])