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"
                    }
                }
            }
        }
    }]
)