Helma mapping limitations

Today I thought it would be nice to have an extendible item object. My plan was to use one db table for the parent-properties and to use referenced tables in which all specialized data should be stored. The easiest way (i thought) would be to use the key of the parent table also as reference for the foreign key on the specialized tables. Sadly it's not possible this way in Helma and it seems, that I have to use another key for that. The documentation says: The _id entry defines the column to use as primary key. Helma requires a single primary key with no other functionality. And it seems that the key used for references also mustn't have another functionality. As for that, the following is not possible:

_id = ITEM_ID
content = object(Content)
content.local = ITEM_ID
content.foreign = CONTENT_F_ITEM