How to map a Map

Question

I tried

@ManyToMany(cascade = CascadeType.ALL)
Map data = new HashMap();

but it produces the error

Answer 1

Well, the error message is pretty clear: Double isn't an entity. If you want to map a collection of basic elements, use the CollectionOfElement annotation (from Hibernate) or the ElementCollection annotation (from JPA 2.0).

So, assuming you're using Hibernate Annotations 3.4, try this:

@CollectionOfElements(targetElement = Double.class)
@org.hibernate.annotations.MapKey(targetElement = String.class)
Map data;

Or, when using generics:

@CollectionOfElements
Map data;

And if you're using Hibernate Annotations 3.5+, prefer the JPA 2.0 annotations:

@ElementCollection(targetClass = Double.class)
@MapKeyClass(String.class)
Map data;

Or, when using generics:

@ElementCollection
Map data;

References

• Hibernate Annotations 3.4 Reference Guide
  • 2.4.6.2.2. Map
  • 2.4.6.2.5. Collection of element or composite elements
• JPA 2.0 specification
  • Section 11.1.12 "ElementCollection Annotation"
  • Section 11.1.28 "MapKeyClass Annotation"

---

Do you know how to customize the "ELEMENT" and "MAPKEY" column names ?

You can fully customize the result. I think the sample below demonstrates everything:

@CollectionOfElements(targetElement = Double.class)
@JoinTable(name = "COLLECTION_TABLE", 
    joinColumns = @JoinColumn(name = "PARENT_ID"))
@org.hibernate.annotations.MapKey(targetElement = String.class, 
    columns = @Column(name = "SOME_KEY"))
@Column(name = "SOME_VALUE")
private Map data;

• The name of the collection table for the Map is defined using the JoinTable
  • The name of the column for the key to the parent is set using a JoinColumn in the JoinTable
• The name of the column for the key of the map is defined in the MapKey
• The name of the column for the value of the map is defined using the Column