[{"uid":"a1"},{"uid":"a2"}]

db.collection.aggregate([{$match:{uid:{$in:["a1","a2"]}}},{"$project":{"array":{"$objectToArray":{"a1":"ref1","a2":"ref2"}},"uid":1,}},{"$project":{"ref":{"$filter":{"input":"$array","as":"elem","cond":{"$eq":["$$elem.k","$uid"]}}},uid:1,},},{"$project":{ref:{"$arrayElemAt":["$ref.v",0]},uid:1,}}])