Found this comment on GitHub; https://github.com/robconery/massive/pull/50 - with the following comment from Rob;
(...) I had this discussion with others (using SubSonic) with, to be honest, this is more code than how I can use to support what I consider in the case of the edge (and not a terribly good database design for beyond many, many associations)
So it seems that composite keys are not supported. And I can fully understand the reasons for this. Fortunately, we control the database, so I believe that we can get around this by adding a surrogate key, and let the current composite key become a unique index.
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