A Document and a BSONObj logically represent the same thing, but the aggregation framework works with Documents instead of BSONObjs, since the former are cheaper to manipulate. A MatchExpression (used in the $match stage) only accepts a BSONObj, via matchesBSON. This means that for the $match stage, we have to convert a Document to a BSONObj to determine if it matches. We should extend the MatchExpression class to be able to match a Document so we can avoid this potentially expensive conversion.