Vertical Inheritance: Missing subclass attributes with joint prefetch

If you use a joint prefetch on a belongs-to relationship whose target is using vertical inheritance, the attributes on the child class are set to null instead of using the results from the database.

It looks like this is happening when the attribute is read in DescriptorColumnExtractor.visitAttribute, it includes the 'p:' prefetch prefix in the data row key that gets built. Later, the data row result is read from a map without the prefix in the key, and is not found. This only happens on a joint prefetch, disjoint and disjointById work correctly.