Reduce an array of elements into an object in MongoDB

Question

I have a MongoDB collection named Venue with elements of type:

{
    venue: "Grand Hall",
    sections: [{
        name: "Lobby",
        drinks: [{
            name: "Vodka",
            quantity: 3
        }, {
            name: "Red Wine",
            quantity: 1
        }]
    }, {
        name: "Ballroom",
        drinks: [{
            name: "Vodka",
            quantity: 22
        }, {
            name: "Red Wine",
            quantity: 50
        }]
    }]
}

I want to calculate the total amounts of each drink for the party. So I want my result to be something like that:

{
    venue: "Grand Hall",
    sections: 2,
    drinks: [{
        name: "Vodka",
        quantity: 25
    }, {
        name: "Red Wine",
        quantity: 51
    }]
}

Answer 1

There are lots of ways to do this. Here's another one using "$reduce" and "$map", etc.

db.Venue.aggregate({
  "$match": {
    "venue": "Grand Hall"
  }
},
{
  "$set": {
    "sections": {"$size": "$sections"},
    "drinks": {
      "$reduce": {
        "input": {  // flatten array of drink objects
          "$reduce": {
            "input": "$sections.drinks",
            "initialValue": [],
            "in": {"$concatArrays": ["$$value", "$$this"]}
          }
        },
        "initialValue": [],
        "in": {
          "$let": {
            "vars": {  // position of drink in $$value, or -1 if not found
              "idx": {"$indexOfArray": ["$$value.name", "$$this.name"]}
            },
            "in": {
              "$cond": [
                {"$eq": ["$$idx", -1]},  // not found?
                {"$concatArrays": ["$$value", ["$$this"]]},  // not found, add object
                {  // found, so update object in $$value by summing quantities
                  "$map": {
                    "input": "$$value",
                    "as": "val",
                    "in": {
                      "$cond": [
                        {"$eq": ["$$val.name", "$$this.name"]},  // right object?
                        {  // yes, update quantity
                          "name": "$$val.name",
                          "quantity": {"$sum": ["$$val.quantity", "$$this.quantity"]}
                        },
                        "$$val"  // wrong object for update, so just keep it
                      ]
                    }
                  }
                }
              ]
            }
          }
        }
      }
    }
  }
})

Try it on mongoplayground.net.