Why can't I reference child entities with part of the parent entities composite key?

The error is correct. I'm guessing that you are using SchemaExport to generate your tables based on the NHibernate mappings, since the error you are receiving sounds like it would occur during the creation of your tables and foreign keys. SchemaExport would produce tables similar to the following (please note the explanations scattered throughout the code).

I've never used nhibernate, but I guess mapping is quite similar the hibernate ones.

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