Find id field in array of objects and return another field of that object

Question

I can't really figure out how to return nested fields within array of objects. Here is my schema:

const chat = new Schema({
    id: {
        type: String,
        required: true
    },
    channels: [
        {
            id: {
                type: String
            },
            messages: [
                {
                    author: String,
                    body: String,
                    created_at: Date,
                    _id: false,
                    avatar: String
                }
            ]
        }
    ]

})

I want to receive 50 channel messages by using chat ID and channel ID from specific range provided by user (0-49, 50-99 and so on).

So in the end I receive array of objects from that channel.

const messages = [{...}, {...}, ...]

Tom Slabbaert · Accepted Answer · 2019-06-12 12:33:04Z

1

Just a different variation of the first answer to actually return ONLY the required channel and not all of them.

db.getCollection("collection").aggregate(
    [
        { 
            "$match" : {
                "id" : chatid
            }
        }, 
        { 
            "$unwind" : "$channels"
        }, 
        { 
            "$match" : {
                "channels.id" : channelid
            }
        }, 
        { 
            "$project" : {
                "messages" : {
                    "$slice" : [
                        "$channels.messages", 
                        0.0, 
                        50.0
                    ]
                }
            }
        }
    ]
);

answered Jun 12, 2019 at 12:33

Tom Slabbaert

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1 Comment

loonytoons Over a year ago

Good catch tom, I edited my answer to reflect the additional match, but I like your use of slicing in the project stage 👍

loonytoons · Accepted Answer · 2019-06-13 02:26:26Z

I'd use the aggregation pipeline for this. I haven't used mongoose but for a basic mongo query it'd look like:

db.getCollection("collection").aggregate(
    [
        { 
            //find the matching documents
            "$match" : {
                "id" : "chatid", 
                "channels.id" : "channelid"
            }
        }, 
        { 
            //split the document into the individual messages
            "$unwind" : "$channels.messages"
        },
        { 
            "$match" : {
                "channels.id" : "channelid"
            }
        }, 
        {
            //limit to 50 (can be after the project stage)
            "$limit" : 50
        }, 
        { 
            //update to just return the message subdoc
            "$replaceRoot" : {
                "newRoot" : "$channels.messages"
            }
        }
    ]
);

For mongoose check the aggregation API or aggregation middleware docs to implement this

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