<class name="Fixture" table="fixture">
           <id name="id">
             <generator class="native"/>
           </id>

          <many-to-one
            name="ground"
            column="ground_id"
            class="Ground"
            not-null="true"/>
          <many-to-one
            name="series"
            column="series_id"
            class="Series"
            not-null="true"/>
          <many-to-one
            name="sessionOfPlay"
            column="session_of_play"
            class="AspectVariation"/>

          <set name="teams" inverse="false" cascade="save-update">
            <key column="fixture_id"/>
            <one-to-many class="FixtureTeam"/>
          </set>
       </class>



       <class name="Team" table="team">
         <id name="id">
           <generator class="native"/>
         </id>
         <many-to-one
           name="teamType"
           column="team_type_id"
           class="TeamType"
           not-null="true"/>

         <set name="series" table="series_team">
           <key column="team_id"/>
           <many-to-many column="series_id"
           unique="true"
           class="Series"/>
         </set>

         <set name="fixtures" inverse="true" cascade="save-update">
          <key column="team_id"/>
          <one-to-many class="FixtureTeam"/>
         </set>
      </class>

      <class name="FixtureTeam" table="fixture_team">
       <composite-id >
          <key-many-to-one name="team" class="Team" column="team_id" />
          <key-many-to-one name="fixture" class="Fixture" column="fixture_id" />
        </composite-id>
      </class>

Team team = (Team)s.load(Team.class,new Integer(17));
  Fixture fixture = new Fixture();
  fixture = (Fixture)s.load(Fixture.class, new Integer(1));

  Set set = new HashSet();
  set.add(team);
  fixture.setTeams(set);

  s.merge(fixture);
  s.getTransaction().commit();

org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of uk.co.planetbeyond.Beans.FixtureTeam.team

   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:195)

   at org.hibernate.tuple.component.AbstractComponentTuplizer.getPropertyValue(AbstractComponentTuplizer.java:87)

   at org.hibernate.tuple.component.AbstractComponentTuplizer.getPropertyValues(AbstractComponentTuplizer.java:93)

   at org.hibernate.tuple.component.PojoComponentTuplizer.getPropertyValues(PojoComponentTuplizer.java:109)

   at org.hibernate.type.ComponentType.getPropertyValues(ComponentType.java:376)

   at org.hibernate.type.ComponentType.replace(ComponentType.java:455)

   at org.hibernate.type.EntityType.replace(EntityType.java:289)

   at org.hibernate.type.CollectionType.replaceElements(CollectionType.java:507)

   at org.hibernate.type.CollectionType.replace(CollectionType.java:574)

   at org.hibernate.type.TypeFactory.replace(TypeFactory.java:505)

   at org.hibernate.event.def.DefaultMergeEventListener.copyValues(DefaultMergeEventListener.java:496)

   at org.hibernate.event.def.DefaultMergeEventListener.entityIsPersistent(DefaultMergeEventListener.java:267)

   at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:240)

   at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84)

   at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:705)

   at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:689)

   at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:693)

   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

   at java.lang.reflect.Method.invoke(Method.java:324)

   at org.hibernate.context.ThreadLocalSessionContext$TransactionProtectionWrapper.invoke(ThreadLocalSessionContext.java:342)

   at $Proxy0.merge(Unknown Source)

   at uk.co.planetbeyond.test.main(test.java:109)

Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class

   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

   at java.lang.reflect.Method.invoke(Method.java:324)

   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:169)

   ... 23 more