Javadoc
INTERNAL:
This method is called from individual mappings during their
initialization. If the mapping is to a class within a TABLE_PER_CLASS
inheritance hierarchy, what this method will do is prepare a selection
query to execute for every child descriptor in the hierarchy.
The selection queries are created by cloning the source mapping,
updating the necessary database fields on the mapping and then
initializing the mapping to create the internal selection query.
This query is then cached where needed using the source mapping's
selection query name as the key.