Hello everybody

I have a question concerning a mapping of Map<@Entity, enum>.

My mapping is specified as:



If I use @ManyToMany (the appropriate association with the OtherEntityClass), I get an exception:




Looks like Hibernate expects an @Entity as the map value too, not only for the key. I couldn't find a way to declare the enum such that it will persist to a column under @ManyToMany, at least not via annotations.

The enum is describing the type of relation between the two entities so there is no reason why to create a new entity just for that.

One way to workaround this is using @CollectionOfElements (omitting the inverseJoinColumn attr in @JoinTable). While this works, I don't think it's the proper solution because @CollectionOfElements relates mainly to embedded fields, not separate Entities and there are several implications - for example, I can't use the mappedBy attr.


Is there a way to directly persist a Map<@Entity, enum> using @ManyToMany?

Thanks,
Yuval

That what you want to do is actually known as 'Adding columns to join tables'
(book Java Persistence with Hibernate chapter 7.2.3).
You may find some related information searching by this therm.

I reviewed that section, thank you. I actually saw that section before but found the following 7.2.4 "Mapping Maps" even more related. The thing is, IMHO all these approaches don't really answer this case. They all have one thing in common: they force you to create a new entity/value type even if it's not needed - thus imposing a concrete design, just to fit the DB schema and satisfy the persistence layer. Isn't that something that Hibernate supposed to completely avoid?

For example, if following the business domain in the book, what if I have an Item and Bids in a Many-to-Many association (each bid can have many items too), describing also if a particular bid did or didn't win a particular item. In the database it would indeed translate to: ITEMS -(n:m)- ITEM_BIDS -(n:m)- BIDS , where ITEM_BIDS will hold the boolean column ITEM_BID_IS_WINNING.

But in the object domain, you don't need to express this relation by creating a new type (either entity or value) - you can also do it in class Item: Map<Bid, Boolean> or in class Bid: Map<Item, Boolean>. If we take the solution of the book, it would require us to create a new type ItemBid (extends Bid?) just for containing that Boolean.

Although one can argue that ItemBid should be created even for one column (can grow etc') I think Hibernate should be able to handle this case too - supporting @OneToMany or @ManyToMany with Maps containing non-mapped values and persisting them to a column in the joinTable.