littlegoblin-63 Error message on startup after "grails clean"

Added mapping. It's strange that normally there are no error messages, and sometimes Grails starts to pine for mappedBy statements.
Note: it seems the framework is satisfied when I add _some_ mappings.
Merge pull request #64 from dewarim/littlegoblin-62

littlegoblin-63 Error message on startup after "grails clean"