Currently, there is no way to load an entity that exists in the database
with a composite ID, if one of the composite ID columns is null.
This behavior is due to this code in ComponentType#hydrate:
Basically, if any field/property in a composite ID is null, Hibernate
assumes the entire ID is null. An entity cannot have a null ID, so it
returns null for the entity result.
I believe that Hibernate does allow a primary key column to be nullable.
TBH, it seems strange to have a property in a composite ID that can be
null. If it can be null, it seems that the property could be removed from
the composite key.
I don't see anything in the spec about a requirement that all composite ID
fields/properties must be non-null. Am I missing something?
The code I referenced above is 13 years old. Does anyone have insight into
why Hibernate does this?