It appears you want to use this second form for your table structure. You did some of the time, but not others, so maybe it's just a mis-copy.

I was under the understanding (perhaps incorrect?) that C:DBI will look for corresponding has_a declerations?

When setting up the relationship we examine the foreign class's has_a() declarations to discover which of its columns reference our class... If no such has_a() declarations can be found, or none link to us, we assume that it is linking to us via a column named after the moniker() of our class. If this is not true you can pass an additional third argument to the has_many() declaration stating which column of the foreign class references us.

(from C:DBI Docs)