How get all objects from an array of Id's in mongoose with express nodejs?

Question

I'm developing a Chat application where I'm saving the conversation list in the DB like this;

Conversations Collection:

{ 
   members: [ "123", "456" ]
   ...rest
}

and User Collection:

{
   _id: ObjectId( "123" ), name: "anyone"
},
{
   _id: ObjectId( "456" ), name: "someone"
}

I want the result to be like:

{
   members: [
              {_id: ObjectId( "123" ), name: "anyone"},
              {_id: ObjectId( "456" ), name: someone"}
            ]
}

I know aggregation with lookup is the rescue but can't find a way how to fetch all ids from that member's array because in the future it can be 20 to 30 ids. If it is a simple field then I'm able to fetch but with an array, I can't.

I have tried this

db.conversations.aggregate([
  {
    "$lookup": {
      "from": "users",
      "localField": "members",
      "foreignField": "_id",
      "as": "members_details"
    }
  }
])

but it returns members_details: [ ]

Matt Oestreich · Accepted Answer · 2021-10-15 14:44:05Z

1

You could use the following query to accomplish what you want. You'll need to use $lookup since you are wanting to gather data from a different collection then the one you are currently querying.

You can check out a live demo here

Here is an updated live demo

Database

db={
  "conversations": [
    {
      members: [
        123,
        456
      ]
    }
  ],
  "users": [
    {
      "_id": 123,
      "name": "foo"
    },
    {
      "_id": 456,
      "name": "bar"
    }
  ]
}

Query

db.conversations.aggregate([
  {
    "$lookup": {
      "from": "users",
      "localField": "members",
      "foreignField": "_id",
      "as": "members"
    }
  },
  {
    $project: {
      _id: 0,
      members: 1
    }
  }
])

Result

[
  {
    "members": [
      {
        "_id": 123,
        "name": "foo"
      },
      {
        "_id": 456,
        "name": "bar"
      }
    ]
  }
]

Update

See new live demo here

New Query

db.conversations.aggregate([
  {
    $unwind: "$members"
  },
  {
    "$lookup": {
      "from": "users",
      "as": "membersFlat",
      "let": {
        memberObjectId: {
          "$toObjectId": "$members"
        }
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $eq: [
                "$$memberObjectId",
                "$_id"
              ]
            }
          }
        }
      ]
    }
  },
  {
    $group: {
      _id: null,
      members: {
        $push: {
          "_id": {
            $first: "$membersFlat._id"
          },
          "name": {
            $first: "$membersFlat.name"
          }
        }
      }
    }
  },
  {
    $project: {
      _id: 0
    }
  }
])

New Result

[
  {
    "members": [
      {
        "_id": ObjectId("124578987898787845658574"),
        "name": "foo"
      },
      {
        "_id": ObjectId("124578986532124578986532"),
        "name": "bar"
      }
    ]
  }
]

edited Oct 15, 2021 at 14:44

answered Oct 15, 2021 at 14:00

Matt Oestreich

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5 Comments

Sandip Dhang Over a year ago

This will not work because in the users collection _id represents as a ObjectId type not a string. already tried this solution not worked. Anyway thanks

Sandip Dhang Over a year ago

I have updated your demo check - mongoplayground.net/p/bfZENqF8s3c DB will be like this

Matt Oestreich Over a year ago

I went ahead and updated to demo again to show how you can use ObjectId. mongoplayground.net/p/T4Cjf3-G9mq

Sandip Dhang Over a year ago

Thanks, @matt but I can't make members array as an ObjectId of the array because it is coming from frontend and directly saved to DB

Matt Oestreich Over a year ago

Check out the update to my answer. See here for live demo mongoplayground.net/p/-P1IcxG4JZZ

turivishal · Accepted Answer · 2021-10-15 14:33:27Z

0

Since your members id is string in conversation collection, you can convert it to object id and the join to users collection,

$addFields to update members array
$map to iterate loop of members array
$toObjectId to convert string object id type to object id type

db.conversations.aggregate([
  {
    $addFields: {
      members: {
        $map: {
          input: "$members",
          in: {
            $toObjectId: "$$this"
          }
        }
      }
    }
  },
  {
    $lookup: {
      from: "users",
      localField: "members",
      foreignField: "_id",
      as: "members"
    }
  }
])

Playground

answered Oct 15, 2021 at 14:33

turivishal

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Comments

Tobias S. · Accepted Answer · 2021-10-15 13:51:48Z

0

You can use a normal .find() like this:

const members = [
    ObjectId('4ed3ede8844f0f351100000c'),
    ObjectId('4ed3f117a844e0471100000d'), 
    ObjectId('4ed3f18132f50c491100000e')
]

const docs = await model.find({
    '_id': { $in: members }
}).exec()

answered Oct 15, 2021 at 13:51

Tobias S.

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2 Comments

Sandip Dhang Over a year ago

I want to query on conversation collection and try to get the user object. How a normal find will work?

Tobias S. Over a year ago

sorry, i misunderstood the question

Dmitriy · Accepted Answer · 2021-10-15 15:49:02Z

0

Install mongoose-autopopulate. And in your model code

import {Schema, model} from 'mongoose';

const modelSchema = new Schema({
   ...
   chats: [{type: Schema.Types.ObjectId, ref: "Chat", autopopulate: true}]
})

modelSchema.plugin(require('mongoose-autopopulate'));
...

answered Oct 15, 2021 at 15:49

Dmitriy

1

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