[Zope] ZPatterns Question

I created a ZClass called "Product" with Dataskin as baseclass. I also added a specialist called ProductManager containing a default rack. Under storage for the rack I selected the newly created Product ZClass. My product ZClass needs to query both a RDBMS and the ZODB for attributevalues. At this point I get lost. I guess i should add an attributeprovider plug-in??? What should go into the fromexpr and attrsexprs properties of the attributeprovider? Should one add an attributeprovider for each attribute of the product? Roché