Question

I need to persist a member with the type Map<Item, Set<Item>>, using JPA2 annotations.
The relation is many to many and Item objects are entities.

Should I create a separate intermediary Entity holding Set<Item> or is a direct mapping possible?

Please advise if some context is missing.

Answer 1

Should I create a separate intermediary Entity holding Set<Item> or is a direct mapping possible?

JPA doesn’t support nested collection relationships (List of Lists, Map of Sets, etc). Here is the relevant section of the specification about Map:

2.7 Map Collections

Collections of elements and entity
relationships can be represented as
java.util.Map collections.

The map key and the map value independently can each be a basic
type, an embeddable class, or an
entity.

…

So yes, use an entity holding the Set<Item> and then map your relation as Map<Item, MyHolder>.

References

• JPA 2.0 specification
  • Section 2.7 “Map Collections”
• JPA Wiki Book
  • Nested Collections, Maps and Matrices