How do you map a "Map" in hibernate using annotations?

Question

Using annotations how do you map a field in an entity which is a "Map" (Hashtable) of String to a given object? The object is annotated and instances of it are already stored in the hibernate databse.

I've found the syntax for definging a map with a simple key and value as such:

<class name="Foo" table="foo">     ...     <map role="ages">          <key column="id"/>          <index column="name" type="string"/>          <element column="age" type="string"/>      </map>  </class> 

And oddly with an entity as the key and a simple type as the value like so:

<class name="Foo" table="foo">     ...   <map role="ages">     <key column="id"/>     <index-many-to-many column="person_id"           class="Person"/>     <element column="age" type="string"/>   </map> </class> <class name="Person" table="person">     ...     <property name="name" column="name"           type="string"/> </class> 

But I don't see how to do this for a simple key to element mapping, and I don't see how to do this using annotations.

Answer 1

You could simply use the JPA annotation @MapKey (note that the JPA annotation is different from the Hibernate one, the Hibernate @MapKey maps a database column holding the map key, while the JPA's annotation maps the property to be used as the map's key).

@javax.persistence.OneToMany(cascade = CascadeType.ALL) @javax.persistence.MapKey(name = "name") private Map<String, Person> nameToPerson = new HashMap<String, Person>(); 

Answer 2

@CollectionOfElements(fetch = FetchType.LAZY) @JoinTable(name = "JOINTABLE_NAME",     joinColumns = @JoinColumn(name = "id")) @MapKey(columns = @Column(name = "name")) @Column(name = "age") private Map<String, String> ages = new HashMap<String, String>();