How to map a Map of Lists of entities?

sbrannen · **Joined:** Fri Dec 16, 2005 2:50 pm **Posts:** 2

Hi,

I'm having a little difficulty figuring out how to map nested collections of entities. In particular, I am trying to map a Map of Lists of persistent entities.

The example Java declaration is as follows:

Code:

private Map<String, List<Entity>> map = new HashMap<String, List<Entity>>();

The mapping is then as follows:

Code:

<map name="map" table="map_of_lists">
  <key column="entity_id" />
  <index column="map_key" type="java.lang.String" />
  <???>
</map>

The "thing to be mapped" (i.e., the value in the java.util.Map) is neither directly my Entity class, nor is it a component or a composite-element. Rather, it is a collection of entities.

Note: I am not asking how to map my Entity class: the mapping for the Entity works fine.

I would be very greatful for tips on what I need to insert for the ??? section to define a proper mapping for a nested collection of entities.

Thanks in advance for any insight you might be able to give me!

Regards,

Sam

p.s. enviroment details follow.

Hibernate version: 3.1.3

Name and version of the database you are using: mysql-5.0.15-win32

tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

There is no direct support for that. The only solution that I can think of that doesn't require you to change your java code is to put a <component> in your map, and the list in the component.

sbrannen · **Joined:** Fri Dec 16, 2005 2:50 pm **Posts:** 2

Hi tenwit,

Thanks for the quick reply.

tenwit wrote:

There is no direct support for that.

I somewhat assumed and feared that. :(

tenwit wrote:

The only solution that I can think of that doesn't require you to change your java code is to put a <component> in your map, and the list in the component.

I also thought that sounded feasible, but looking at the DTD below, you can see that <component> is not a supported sub-element of <map>.

http://hibernate.sourceforge.net/hibern ... ng-3.0.dtd

Here are the supported sub-elements:

(element|one-to-many|many-to-many|composite-element|many-to-any)

If you read further, you will notice the following information for the composite-element, with emphasis on "Composite elements may not hold nested collections":

Quote:

A composite element allows a collection to hold instances of an arbitrary
class, without the requirement of joining to an entity table. Composite elements
have component semantics - no shared references and ad hoc null value semantics.
Composite elements may not hold nested collections.

So, it does not appear that I can achieve a mapping of nested collections. Instead, I assume I will actually have to modify my Java code.

Do you see any other way around this issue? Am I missing something?

Thanks,

Sam

tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

Looks like you need to use an entity in the <map>, and the entity would have just an <id>, optionally a <many-to-one> back to your existing (parent) entity, and the <list> that you need.

I tried figuring out what the DB schema that implements your initial idea would look like, and I don't think that it's possible. You need a table to hold the list, and that table needs the id of the parent entity, the list index, and whatever data you want to store. And probably a surrogate key, for convenience. All of that means that you need an entity, so it looks like the hibernate restrction that you can't put a collection in a colleciton isn't because hibernate can't handle it, but rather that RDBMSs can't handle it.