public class Attribute {
   private Long id;

   private String name;

   private String value;

   private VocabularyElement parent;

   // This map will result in its own table using the hbm.xml file listed below.
   // What annotations can I use instead of the hbm.xml file?
   private Map<String, String> extensionAttributes = new HashMap<String, String>();

   // Constructors, getters, setters
   // ...
}

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
   <class name="com.tracetracker.mds.adm.domain.Attribute">
      
      <id name="id" access="field">
         <generator class="native"/>
      </id>
      
      <natural-id>
         <property name="name" access="field"/>
         <many-to-one name="parent" class="com.tracetracker.mds.adm.domain.VocabularyElement"/>      
      </natural-id>
      
      <property name="value" type="text"/>
      
      <!-- This tag creates the extra table. -->
      <map name="extensionAttributes" table="Attribute_Attr"
            order-by="attr_name asc" lazy="false" access="field">
         <key column="id"/>
         <map-key column="attr_name" type="string"/>
         <element column="attr_value" type="string"/>
      </map>
            
   </class>
</hibernate-mapping>

create table Attribute (
   id int8 not null,
   name varchar(255) not null,
   value text,
   parent int8 not null,
   primary key (id),
   unique (name, parent)
);

create table Attribute_Attr (
   id int8 not null,
   attr_name varchar(255) not null,
   attr_value varchar(255),
   primary key (attr_name, id)
);

alter table Attribute add constraint FK7839CA7C7FB02B5A foreign key (parent) references VocabularyElement;
alter table Attribute_Attr add constraint FK4F4769F4A36A3D31 foreign key (id) references Attribute;