JAXB Unmarshalling Entities - References null

I have 2 mapping classes (analogue to JPA classes):

@XmlType(propOrder = {"name", "children", })
@XmlRootElement(name = "a")
@XmlAccessorType( XmlAccessType.PROPERTY)
public class A implements Serializable {

 private String name;
 private List<B> children;

 @XmlElement(name = "metadatum")
 public List<B> getChildren(){
    return children;
 }

 ...
}

@XmlRootElement(name = "metadatum")
@XmlType(propOrder = {"name"})
public class B implements Serializable{

 private String name;
 private A parent;

 ...
}

A and B are in a OneToMany relation. The XML should look like this:

<A>
  <B></B>
  <B></B>
</A>

If I unmarshal the xml, map it to my JPA classes and persist it to my database everything is stored correctly except my references. This means that B is stored without a foreign key to A in the database.