aggregation: Should have a $zip operator to combine arrays

Syntax

{

    $zip: {

        inputs: <array of expressions>

        defaults: <array of expressions>   

        useLongestLength: <bool - default false, cannot specify defaults if false>

    }

}

Example

Input

{_id: 0, evens: [0, 2, 4, 6], odds: [1, 3, 5]}

{_id: 1, evens: [4], odds: [1, 3]}

Pipeline

db.coll.aggregate([{

    $project: {

        z: {

            $zip: {

                inputs: ["$evens", "$odds"],

                defaults: [0, 1],

                useLongestLength: true

            }

        }

    }

}])

Output

{_id: 0, z: [[0, 1], [2, 3], [4, 5], [6, 1]]}

{_id: 1, z: [[4, 1], [0, 3]]}

Additional Notes

• Uses length of shortest array, unless useLongestLength is true. If useLongestLength is true, but defaults is not specified, will use null to fill in.
• The use of null here is consistent with our parsing of array literals (e.g. [0, "$field"] will become [0, null] if 'field' does not exist).
• 'defaults' cannot be specified unless useLongestLength is true.
• If any input is null, will return null. $map returns null if its input is nullish.
• Will error if any input is not an array (and is not nullish).
• Will error if 'defaults' is specified without 'useLongestLength', or is not the same length as 'inputs'.