Query in nested document in mongodb

Question

Here is my mongoose schema:

const chapterSchema = mongoose.Schema({
title: {
    type: String
},
number: {
    type: Number
}
})

const bookSchema = mongoose.Schema({
    title: {
        type: String
    },
    author: {
        type: String
    },
    chapters: [chapterSchema]
})

const Book = mongoose.model('Book', bookSchema)
module.exports = Book

Here is my database:

{
"_id": "5f56f8e66a7eee227c1acc0a",
"title": "Book1",
"author": "Author1",
"chapters": [{
    "_id": "5f56fa47a78fbf03cc32d16d",
    "title": "Chapter1",
    "number": 1
}, {
    "_id": "5f56fad10820300de031317f",
    "title": "Chapter2",
    "number": 2,
}]
}

I want to find a specific chapter from the chapters array by it's id. So, I wrote this code:

router.get('/:id', async function (req, res) {
try {
    const id = req.params.id
    await Book.findById(id, function (err, chapter) {
        res.render('chapter.hbs', {
            chapter: chapter
        })
    })
} catch (error) {
    res.redirect('/')
}
})

According to the chapter id, I want this result:

{
    "_id": "5f56fa47a78fbf03cc32d16d",
    "title": "Chapter1",
    "number": 1
}

or

{
    "_id": "5f56fad10820300de031317f",
    "title": "Chapter2",
    "number": 2,
}

What should I do to find a specific chapter using it's id?

Answer 1

You are searching on Book using findById of chapters Id, it will not give result since it is not Book Id,

You will need to use $elemMatch for this

check this Querying for Object in Mongoose Sub Array for reference

Answer 2

You can do this using the aggregate function.

router.get('/:id', async function (req, res) {
  try {
    const id = req.params.id
    await Books.aggregate([
      {
        $project: {
          title: 1,
          author: 1,
          chapters: {
            $filter: {
              input: "$chapters",
              as: "chapter",
              cond: {
                $eq: [
                  "$$chapter.id", id
                ]
              }
            }
          }
        }
      }
    ])
  } catch (error) {
    res.redirect('/')
  }
})

Answer 3

You can use aggregate() function,

• $match your condition
• $filter to get specific chapter id
• $replaceWith to replace chapter in root

const id = mongoose.Types.ObjectId(req.params.id);

let chapter = await Book.aggregate([
  { $match: { "chapters._id": id } },
  {
    $replaceWith: {
      $arrayElemAt: [
        {
          $filter: {
            input: "$chapters",
            cond: { $eq: ["$$this._id", id] }
          }
        },
        0
      ]
    }
  }
]);

console.log(chapter);

Playground