How to count items in list of lists

Question

How i can find length of each item in a \"mol\"list:

For example if I am looking for \"versionA\" : \"2.1.2\" I expect ot get following result:

Answer 1

You can $match to the version field and $map which iterates the mol and $size to calculate the the length of data field.

db.collection.aggregate(
    [{
        $match: {
            "versionA": "2.1.2"
        }
    }, {
        $project: {
            _id: 0,
            "project": 1,
            "scene": 1,
            "mol": {
                $map: {
                    input: "$mol",
                    as: "mo",
                    in: {
                        $size: "$$mo.data"
                    }
                }
            }
        }
    }]
)

Answer 2

There is another way of doing it as well:

First you $map each element of "mol", inside that you reduce each element of mol to the size of it's inside array using $reduce.

db.stack.aggregate([
    {
        $match: { "versionA": "2.1.2" }    
    },
    {
        $project: {
            _id: 0,
            "project": 1,
            "scene": 1,
            mol: {
                $map: {
                  input: "$mol",
                  as: "ob",
                  in: {
                      $reduce: {
                            input: "$$ob.data",
                            initialValue: 0,
                            in: { $add: ["$$value", 1] }  
                      }
                  }
                }
            }
        }
    }
]);