MongoDB nested aggregation

Question

So, I have two collections User and Book, and I want to aggregate them to receive an output (shown at the end of the post) for a specific User ID.

below are the collections:

User

Each document contains User ID, Name of the User and an array containing ID of a document from Book collection and a Boolean property read.

[
  {
    _id: ObjectId('adfa2sd5'),
    name: "Mia",
    books: [
        {
            bid: "154854",
            read: true
        },
        {
            bid: "5475786",
            read: false
        }
    ]

  },

  {
    _id: ObjectId('uai5as5a'),
    name: "Jack",
    books: [
        {
            bid: "5475786",
            read: true
        }
    ]

  }
]

Book

Each document possesses a book ID and name of the book.

[
  {
    _id: ObjectId('154854'),
    name: "The Great Gatsby"
  },

  {
    _id: ObjectId('5475786'),
    name: "Frankenstein"
  },
]

Output:

The output contains the User ID, along an array book_list which contains detail of each book (id, name) from the documents of Book collection based on the books.bid from User document and read field which was along books.bid.

[
  {
    _id: ObjectId('adfa2sd5'),
    book_list: [
        {
            _id: ObjectId('154854'),
            name: "The Great Gatsby",
            read: true
        },
        {
            _id: ObjectId('5475786'),
            name: "Frankenstein",
            read: false
        }
    ]

  }
]

Answer 1

Here's one way you could do it.

db.users.aggregate([
  {
    "$match": {
      "_id": ObjectId("fedcba9876543210fedcba98")
    }
  },
  {
    "$lookup": {
      "from": "books",
      "localField": "books.bid",
      "foreignField": "_id",
      "as": "bookList"
    }
  },
  {
    "$set": {
      "books": {
        "$map": {
          "input": "$books",
          "as": "book",
          "in": {
            "$mergeObjects": [
              "$$book",
              {
                "name": {
                  "$getField": {
                    "field": "name",
                    "input": {
                      "$first": {
                        "$filter": {
                          "input": "$bookList",
                          "cond": {"$eq": ["$$this._id", "$$book.bid"]}
                        }
                      }
                    }
                  }
                }
              }
            ]
          }
        }
      }
    }
  },
  {"$unset": ["bookList", "name"]}
])

Try it on mongoplayground.net.