How to merge multiple facet results into a list of objects with one attribute

Question

The situation:

In my mongodb aggregate pipeline I have a $facet stage which produces multiple output fields e.g.: pipe_1 and pipe_2.

The result has the following form:

{
  pipe_1: [{"key": "a"}, {"key": "b"}, {"key": "c"}, {"key": "d"}],
  pipe_2: [{"key": "a"}, {"key": "B"}, {"key": "d"}]
}

Wanted result

My question is now, how do I get the following result from it:

[
  {"key": "a"},
  {"key": "d"}
]

I want only the keys, that are in both list and no duplicated should be in the result. Preferably I would like to have an answer without any slow group stages and it must work for more than two list.

Thank you in advance.

Answer 1

You can use $setIntersection operator:

db.collection.aggregate([
    {
        $project: {
            result: {
                $setIntersection: [ "$pipe_1", "$pipe_2" ]
            }
        }
    }
])

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