'Reverse-lookup of virtual references which are declared for
the respective lookup-table as:
configure(tablename,
referenced_by = [(tablename, fieldname), ...],
& in the table with the fields(auth_user only current example) as:
configure(tablename,
references = {fieldname: tablename,
@param field: the Field
@returns: the name of the referenced table'

@classmethod
def virtual_reference(cls, field):

    if (str(field.type) == 'integer'):
        config = current.model.config
        (tablename, fieldname) = str(field).split('.')
        this_config = config[tablename]
        if this_config:
            references = this_config.get('references')
            if ((references is not None) and (fieldname in references)):
                return references[fieldname]
        key = (tablename, fieldname)
        for tn in config:
            referenced_by = config[tn].get('referenced_by')
            if ((referenced_by is not None) and (key in referenced_by)):
                return tn
    return None
