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>();