Database
[{"docId":"x1"},{"docId":"x2"},{"docId":"x3"},{"docId":"x4"},{"docId":"x5"},{"docId":"x434"},{"docId":"xn"}]
Query
db.collection.aggregate([{$match:{"docId":"x434"}},{"$unionWith":{"coll":"collection","pipeline":[/** exclude x434 to avoid duplicate*/{$match:{"docId":{$ne:"x434"}}}/** put your other queries here*/]}}])