Count field values dynamically in mongo aggregation

Question

I have three collections listings, listingsCollections and profile. I used aggregation on listingsCollections and matched collectionID from listings. I used another lookup to add profile info to listings. My lookup response is like this

{
    //listingsCollections fields
    collectionID: 'abc', 
    collectionName: 'xyz',
    listings: [
        {
            // listings fields
            type: 'free',
            listingName: 'listing one',
            // profile fields
            ownerInfo: {
                name: 'user 1'
            }
        },
        {
            type: 'free',
            listingName: 'listing three',
            ownerInfo: {
                name: 'user 1'
            }
        },
        {
            type: 'paid',
            listingName: 'listing two',
            ownerInfo: {
                name: 'user 1'
            }
        },
    ]
}

I want to calculate type with count and total listings count.I don't want to pass hard coded type values in $addFields with $cond.

This is the disired output i am looking for.Is there any way to achieve this

{
    collectionID: 'abc', 
    collectionName: 'xyz',
    listingCount: {
        free: 2,
        paid: 1
    },
    totalListings: 3,
    listings: [
        {
            type: 'free',
            listingName: 'listing one',
            ownerInfo: {
                name: 'user 1'
            }
        },
        {
            type: 'free',
            listingName: 'listing three',
            ownerInfo: {
                name: 'user 1'
            }
        },
        {
            type: 'paid',
            listingName: 'listing two',
            ownerInfo: {
                name: 'user 1'
            }
        },
    ]
}

Answer 1

To create an object with unique keys, we need to use $arrayToObject + $objectToArray operators.

Explanation:

input:
[{"type": "free"}, {"type": "free"}, {"type": "paid"}]
--------------- $objectToArray --------------------
[{"k": "type", "v": "free"}, {"k": "type", "v": "free"}, {"k": "type", "v": "paid"}]
--------------- $arrayToObject --------------------
{"free": 2, "paid": 1} // since objects cannot have same key for twice

Try this one:

db.collection.aggregate([
  {
    $addFields: {
      listingCount: {
        $arrayToObject: {
          $map: {
            input: "$listings.type",
            as: "type",
            in: {
              k: "$$type",
              v: {
                $size: {
                  $filter: {
                    input: "$listings.type",
                    cond: {
                      $eq: [
                        "$$this",
                        "$$type"
                      ]
                    }
                  }
                }
              }
            }
          }
        }
      },
      totalListings: {
        $size: "$listings"
      }
    }
  }
])

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