hibernate map<key, set<value>>

Question

I have the following tables:

@Entity
@Table(name = "events")    
Event
    --id
    --name

@Entity
@Table(name = "state")    
State
    --id
    --name

@Entity
@Table(name = "action")    
Action
    --id
    --name
@Entity
@Table(name = "state_event_action")
    StateEventAction
--id
--state_id
--event_id
--action_id

I would like in state class to get map<key, set<value>> of Map<Event, set<StateEventAction>>

How can I do it in Hibernate?

Answer 1

I would like in state class to get map<key, Set<value>> of Map<Event, Set<StateEventAction>>

Hibernate does not support collection of collections such as List of Lists, Map of Sets, etc out of the box. But you could implement your own UserCollectionType to add support for this kind of data structure. This blog post shows how to do so using the MultiMap implementation from Apache commons.

My suggestion would be to use a similar approach, but maybe to prefer the generified Multimap from Google Guava.

Answer 2

If you want to recieve Map of sets, it means that there are several actions for each (state_id, event_id) actions. So you have wrong entity's mappings. It should be

@Entity 
@Table(name = "state_event_action") 
StateEventAction 
--id 
--state_id 
--event_id 
--Set<action> actions

In this case you can write :

@Entity @Table(name = "state")     
State 
    --id 
    --name 
 Map<Event,StateEventAction> eventActions;