union two collections in mongodb with one query

Question

I have one collection named USER

{"_id" => "id1", ...}

and another collection named CONTACT

{"_id" => "id2", ...}

now i have an array

[id1, id2]

Can I get two data with one query?

Answer 1

You can use the Aggregation Framework $setUnion operator for that.

db.collection.aggregate(
   [
     { $project: { id1:1, id2: 1, allValues: { $setUnion: [ "$id1", "$id2" ] } } }
   ]
)