Populate + Aggregate in Mongoose

Question

I have two Mongoose models: one for transactions and the other one for the tags associated with them. In order to implement some reports, I need aggregate code like this:

Transaction.aggregate([
  { $unwind: '$tags' },
  {
    $group: {
      _id: '$tags',
      amount: {
        $sum: '$amount'
      }
    }
  }
])

Question

This produces output containing _id and amount. Now, I'd like to populate the other fields (e.g. name) from the model, keeping the calculated amount column. Can I do that within a simple populate?

Edit

The schemas for the models I'm describing:

var TransactionSchema = new Schema({
  description: {
    type: String,
    trim: true
  },
  amount: {
    type: Number,
    required: 'Forneça um valor',
  },
  date: {
    type: Date,
    required: 'Forneça uma data',
    default: Date.now
  },
  fromOfx: {
    type: Boolean,
    default: false
  },
  created: {
    type: Date,
    default: Date.now
  },
  correlated: {
    type: Boolean,
    default: false
  },
  tags: [{
    type: Schema.Types.ObjectId,
    ref: 'TransactionTag'
  }],
  correlates: [{
    type: Schema.Types.ObjectId,
    ref: 'Transaction'
  }],
  user: {
    type: Schema.Types.ObjectId,
    ref: 'User'
  }
});

var TransactionTagSchema = new Schema({
  name: {
    type: String,
    required: 'Forneça um nome',
    trim: true
  },
  description: {
    type: String,
    trim: true
  },
  amount: {
    type: Number
  }
});

Answer 1

You can populate an aggregation after you fetched the data from the MongoDB. This will look something like this:

// Your aggregate query from your question
Transaction.aggregate([{
                            $unwind: '$tags'
                        }, {
                            $group: {
                                _id: '$tags',
                                amount: {
                                    $sum: '$amount'
                                }
                            }
                        }])
    .exec(function(err, transactions) {
        // Don't forget your error handling
        // The callback with your transactions
        // Assuming you are having a Tag model
        Tag.populate(transactions, {path: '_id'}, function(err, populatedTransactions) {
            // Your populated translactions are inside populatedTransactions
        });
    });