When setting up the relationship we examine the foreign class's has_a()
       declarations to discover which of its columns reference our class.
       (Note that because this happens at compile time, if the foreign class
       is defined in the same file, the class with the has_a() must be defined
       earlier than the class with the has_many(). If the classes are in dif-
       ferent files, Class::DBI should be able to do the right thing). 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.