public class [b]MutualRefEntityADo[/b] extends AbstractDo {
    private MutualRefEntityBDo b;
    private Set<MutualRefEntityBDo> mutualRefEntityBs = new HashSet<MutualRefEntityBDo>();

    public MutualRefEntityADo() {
    }


    public MutualRefEntityBDo getB() {
        return b;
    }
    public void setB(MutualRefEntityBDo value) {
        b = value;
    }

    public Set<MutualRefEntityBDo> getMutualRefEntityBs() {
        return mutualRefEntityBs;
    }
    public void setMutualRefEntityBs(Set<MutualRefEntityBDo> value) {
        mutualRefEntityBs = value;
    }
}


[b]Mapping for entity A:[/b]
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping default-cascade="save-update,merge,refresh">

    <class name="MutualRefEntityADo" table="MUTUAL_REF_ENTITY_A" optimistic-lock="version">
        <id name="id" type="long" column="ID" unsaved-value="0">
            <generator class="sequence">
                <param name="sequence">SEQ_MUTUAL_REF_ENTITY_A_id</param>
            </generator>
        </id>

        <version  column="VERSION_" name="version_" access="property" unsaved-value="null"/>
        <many-to-one class="MutualRefEntityBDo" column="B_ID" name="b"/>
        <set inverse="true" lazy="true" name="mutualRefEntityBs">
            <key column="A_ID"/>
            <one-to-many class="MutualRefEntityBDo"/>
        </set>
    </class>

</hibernate-mapping>


public class [b]MutualRefEntityBDo [/b]extends AbstractDo {
    private MutualRefEntityADo a;
    private Set<MutualRefEntityADo> mutualRefEntityAs = new HashSet<MutualRefEntityADo>();

    public MutualRefEntityBDo() {
    }


    public MutualRefEntityADo getA() {
        return a;
    }
    public void setA(MutualRefEntityADo value) {
        a = value;
    }

    public Set<MutualRefEntityADo> getMutualRefEntityAs() {
        return mutualRefEntityAs;
    }
    public void setMutualRefEntityAs(Set<MutualRefEntityADo> value) {
        mutualRefEntityAs = value;
    }
}


[b]Mapping for entity B:[/b]
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping default-cascade="save-update,merge,refresh">

    <class name="MutualRefEntityBDo" table="MUTUAL_REF_ENTITY_B" optimistic-lock="version">
        <id name="id" type="long" column="ID" unsaved-value="0">
            <generator class="sequence">
                <param name="sequence">SEQ_MUTUAL_REF_ENTITY_B_id</param>
            </generator>
        </id>

        <version  column="VERSION_" name="version_" access="property" unsaved-value="null"/>
        <many-to-one class="MutualRefEntityADo" column="A_ID" name="a"/>
        <set inverse="true" lazy="true" name="mutualRefEntityAs">
            <key column="B_ID"/>
            <one-to-many class="MutualRefEntityADo"/>
        </set>
    </class>

</hibernate-mapping>

public void testMutualReferencing() throws Exception {
        MutualRefEntityADo mreA = new MutualRefEntityADo();
        MutualRefEntityBDo mreB = new MutualRefEntityBDo();
        mreA.setB(mreB);
        mreA.getMutualRefEntityBs().add(mreB);
        mreB.setA(mreA);
        mreB.getMutualRefEntityAs().add(mreA);
        session.save(mreA);
        Long mreBId = (Long) session.save(mreB);

[b]        commitAndRestartTx(); //just flushes session, commites transaction, clears session and starts new transaction - exception is thrown here![/b]
        MutualRefEntityBDo longMreB = (MutualRefEntityBDo) session.load(MutualRefEntityBDo.class, mreBId);
        assertNotNull(longMreB);

    }