<hibernate-mapping>
   <class name="FlexAttribute" table="FLEX_ATTRIBUTE">
      <composite-id>
         <key-property name="objectKey" type="java.lang.Long">
            <column name="OBJECT_KEY" not-null="true" />
         </key-property>
         <key-property name="objectType" type="string">
            <column name="OBJECT_TYPE" not-null="true" />
         </key-property>
      </composite-id>
                <property name="stringAttribute1" type="string">
                        <column name="STRING_ATTRIBUTE1" length="1024" not-null="false" />
                </property>
   </class>
</hibernate-mapping>
<hibernate-mapping>
   <class name="MyObject" table="MY_OBJECT">
      <id name="myObjectKey" type="java.lang.Long">
         <column name="MY_OBJECT_KEY" scale="0" />
         <generator class="sequence">
            <param name="sequence">MY_OBJECT_SEQ</param>
         </generator>
      </id>
      <one-to-one name="flexAttribute" class="FlexAttribute">
         <formula>MY_OBJECT_KEY</formula>
         <formula>'1'</formula>
      </one-to-one>
   </class>
</hibernate-mapping>

public class FlexAttribute implements Serializable {
    private String objectType;
    private Long objectKey;
    private String stringAttribute1;
   
    // getters and setters
}

public class MyObject implements Serializable {
    private Long myObjectKey;
    private FlexAttribute flexAttribute;

    //getters and setters
}