Mongoose number of documents referenced in a collection

April 13, 2022

I have 2 collections

User:

email: { type: String, required: true, unique: true },
team: { type: ObjectId, ref: 'team', required: true},

And Team:

 name: { type: String, required: true },
 active: { type: Boolean, required: true, default: true},

I want to get all the teams and the number of users in that team, How would I approch this using Mongoose ?

>Solution :

Use the Teams model and perform an aggregate query.

First we join all users from the users collection. Then we create an extra field called user_amount in which we store the size of the users array. In the last step we remove the users property but if you need this array just remove the last step of this pipeline.

Playground

Teams.aggregate([
  {
    "$lookup": {
      "from": "users",
      "localField": "_id",
      "foreignField": "team",
      "as": "users"
    }
  },
  {
    "$addFields": {
      "user_amount": {
        "$size": "$users"
      }
    }
  },
  {
    "$unset": "users"
  }
])