db={"CollA":[{"_id":"1","name":"Bob"},{"_id":"2","name":"John"},{"_id":"3","name":"Will"}],"CollB":[{"_id":"1","foo":{"arr":["1","3"]}}]}

db.CollB.aggregate([{"$lookup":{"from":"CollA","localField":"foo.arr","foreignField":"_id","as":"collALookup"}},{"$unwind":"$collALookup"},{"$replaceRoot":{"newRoot":"$collALookup"}}])