I want to do an HQL inner join from one table to another. The join is on two fields which don't actually have a mapping relationship defined for those fields. Think of it as an arbitrary join that I only want to do for this particular query, but I don't require a relationship mapped for it in their mapping files.

The thing is, Hibernate then bitches about this and the only way round it is to define a one-to-one mapping and the join works. Is this expected behaviour?

yep, this is the expected behaviour from HQL. Keep in mind that HQL operates with persistent objects and their relationships not with tables, columns or foreign keys. This implies that every property or association you want to use in your HQL query must be mapped. But you can always use plain SQL in your queries instead of HQL - check the docs.

HTH,
radu