36. A method for responding to an object call using a mapping unit, the method comprising the steps of: determining the type of object call; setting up an access plan data structure according to flags settings; creating a command statement for accessing relational system by retrieving a base SELECT statement from ClassInfo, testing whether any predicate has been specified; and translating a specified predicate and appending the predicate as a WHERE clause, if a predicate has been specified; issuing the command statement on relational system, the command statement including the SELECT statement having a WHERE clause, if any, against a database of the relational system; and determining whether more rows are available from the database; if more rows are available from the database, fetching a next row and creating an instance of a top-level object; setting attribute values from corresponding column values; creating required foreign key entries and associating them with target class structures; determining whether there are query objects from the subclasses; and if there are additional query objects from the subclasses, repeat the steps of creating command statement for accessing relational system and issuing the command statement on relational system for the objects of subclasses.
39. The method of claim 36 wherein the step of processing the data from the relational system to provide it to the object-oriented system includes the steps of: creating a SELECT statement and a WHERE clause using the foreign keys for each referenced target class; retrieving rows and creating target objects and linking them with referencing complex attributes; creating a foreign key entry for each complex attribute of the target class; and returning a list of top-level objects to the application.