Formatting data after $facet aggregation in mongodb

Question

First of all, I've gone through several similar questions before posting this one and none have seemed to be accurate to my needs or I haven't been able to interpret them in a way that would work for me.

I am working with aggregation in MongoDB (with NodeJS and Mongoose) to perform some pagination and also give some data like total and average.

This is my pipeline so far:

[
    { 
      $match: { 
        // Some filtering criteria here
      }
    },
    { $facet: {
        metadata: [ 
          { $count: 'total' }
        ],
        avg: [
          {
            $group: {
              _id: null,
              avg_price: {
                $avg: "$price"
              }
            }
          }
        ],
        data: [ 
          { $sort: { createdDate: -1 }}, 
          { $skip: skip || 0 },
          { $limit: limit } 
        ]
      } 
    }
  ] 

Which is giving me an output with the following structure:

[
    {
      "metadata": [
        {
          "total": 14
        }
      ],
      "avg": [
        {
          "_id": null,
          "avg_price": 936711.3571428572
        }
      ],
      "data": [
        // the returned data according to $match, $sort, $skip and $limit
      ]
    }
]

I have to send that data to the front end but that structure is not suitable for my needs. I am using GraphQL and I would prefer so send something like the following (without that array-object-array kind of nesting):

 {
    total: 14,
    avg_price: 936711.3571428572,
    data: [
      // the returned data according to $match, $sort, $skip and $limit
    ]
  }

I could indeed have some Javascript logic to extract that data from the aggregation result and generate the expected output but it would required dirty code like:

avg_price: aggr_result[0].avg[0].avg_price

And I want to avoid that.

I was wondering what would be the MongoDB way to to this kind of formatting in the pipeline.

Thanks for your time.

Answer 1

Just use one $project stage at the end of the pipeline

[
  { "$match": { ... }},
  { "$facet": {
      "metadata": [
        { "$count": "total" }
        ],
      "avg": [
        { "$group": {
          "_id": null,
          "avg_price": { "$avg": "$price" }
        }}
      ],
      "data": [
        { "$sort": { "createdDate": -1 }},
        { "$skip": skip || 0 },
        { "$limit": limit }
      ]
    }
  },
  { "$project": {
    "total": { "$arrayElemAt": ["$metadata.total", 0] },
    "avg_price": { "$arrayElemAt": ["$avg.avg_price", 0] },
    "data": 1,
  }}
]