Collection-agnostic aggregations that get their documents from a document-generating stage like $documents are sent to the server with aggregate: 1. There is currently no way to do this for a pipeline within a $unionWith stage, which will fail with a parse error if the specified collection is 1.

BSON field '$unionWith.coll' is the wrong type 'int', expected type 'string'

This ticket should enable pipelines like:

{
    aggregate: 1,
        pipeline:
            [
                {$documents: [{a: 1}]},
                {
                    $unionWith: {
                        coll: 1,
                        pipeline: [
                            {$documents: [{a: 2}]},
                        ]
                    }
                },
            ],
        cursor: {}
}