Hi Giovanni,
the general problem is that the indexing system requires a single term to unequivocally identify the entity.

So if in the database your composite id is made of two strings (for example), you need to be able to provide a bridge which transforms the pair into a keyword; Unfortunately not knowing what you might store in strings it is not safe for Hibernate Search to do this mapping automatically.

The @DocumentId strategy should work, could you describe your error, or even better provide a full example?