How to get only document from sub-document's array in mongoose

Question

I have a Chapter schema like this:

const ChapterSchema = new Schema({
    title: {
        type: String,
        required: [true, 'Chapter title is required']
    },
    topics: { type: [TopicSchema] }
})

So, there is a topics array as sub-documents of Chapter. I want to get a particular topic by its _id from Chapter. For that I've tried this query below:

let data = await Chapter.findOne({ "topics._id": _id })
return res.json(data)

But it returns a whole chapter of this topic with topic sibling like this:

{
    "_id": "5e504271ee36f61ba8d76f37",
    "title": "Roshayon Chapter 2",
    "topics": [
        {
            "_id": "5e52bdf994b60b4c540cab33",
            "title": "topic 4",
            "intro": "<p><b>This text is bold</b></p><p><i>This text is italic</i></p><p>This is<sub> subscript</sub> and <sup>superscript</sup></p>"
        },
        {
            "_id": "5e52bdf994b60b4c540cab34",
            "title": "topic 5",
            "intro": "<p><b>This text is bold</b></p><p><i>This text is italic</i></p><p>This is<sub> subscript</sub> and <sup>superscript</sup></p>"
        }
    ]
}

I don't need whole chapter as above. I just need a single topic object which I am looking for by its id. How can I able to get

Expected result:

        {
            "_id": "5e52bdf994b60b4c540cab34",
            "title": "topic 5",
            "intro": "<p><b>This text is bold</b></p><p><i>This text is italic</i></p><p>This is<sub> subscript</sub> and <sup>superscript</sup></p>"
        }

Ashish Modi · Accepted Answer · 2020-02-23 20:18:28Z

1

You need to use $elemMatch while projecting so it gives us the matching array rec ord.

something like this should work

let data = await Chapter.findOne({ "topics._id": _id }, {_id: 0, topics: {$elemMatch: {_id: _id}}});

If you need to get the just object, you can use aggregate and use following query

await Chapter.aggregate([
    {$match: {'topics._id': "5e52bdf994b60b4c540cab33"}},
    {$project: {
        topics: {$filter: {
            input: '$topics',
            as: 'topic',
            cond: {$eq: ['$$topic._id', '5e52bdf994b60b4c540cab33']}
        }},
        _id: 0
    }}
]).unwind("topics").exec()

Hope it helps.

edited Feb 23, 2020 at 20:18

answered Feb 23, 2020 at 19:46

Ashish Modi

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4 Comments

Anwarul Islam Over a year ago

It returns this: {"topics": []}

Ashish Modi Over a year ago

apologies, updated my answer. removed the extra topics which wasn't required. Please try now

Anwarul Islam Over a year ago

Now, It's working but the problem is I'm getting the single topic object in topics array. Can I get only that topic?

Ashish Modi Over a year ago

@AnwarulIslam this might be the way aggregate is being used. I am not too familiar with mongoose. I have tried that query directly on mongod and it works fine. Updated it as per this - mongoosejs.com/docs/api/aggregate.html Try now.

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