It is a base class for ResultSets providing just one method: recur-
sive_update which works just like update_or_create but can recursively
update or create data objects composed of multiple rows. All rows need
to be identified by primary keys - so you need to provide them in the
update structure (unless they can be deduced from the parent row - for
example when you have a belongs_to relationship).  If not all colums
comprising the primary key are specified - then a new row will be cre-
ated, with the expectation that the missing columns will be filled by
it (as in the case of auto_increment primary keys).
