class A {
 public virtual long Id { get; set;}
 public virtual B LinkToB { get; set; }
}

class B {
 public virtual long Id { get; set; }
 public virtual A LinkToA { get; set;}
 public virtual string NameB {get ;set;}
}

<hibernate-mapping>
  <class name="A" table="a">
    <id name="Id" column="id" type="long" unsaved-value="-1">
      <generator class="sequence">
        <param name="sequence">a_id_seq</param>
      </generator>
    </id>
    <one-to-one name="LinkToB" class="B" property-ref="LinkToA" cascade="all" constrained="false" />
  </class>
</hibernate-mapping>

<hibernate-mapping>
  <class name="B" table="b">
    <id name="Id" column="id" type="long" unsaved-value="-1">
      <generator class="sequence">
        <param name="sequence">b_id_seq</param>
      </generator>
    </id>
    <many-to-one name="LinkToA" class="A" column="a_id" unique="true" not-null="true" />
    <property name="NameB" type="string">
      <column name="name" not-null="true" />
    </property>
  </class>
</hibernate-mapping>

public void SaveObj()
{
 try
 {
  session.BeginTransaction();
  session.Save(a);
  session.Flush();
  session.Commit();
 } catch (Exception ex)
 {
  session.Rollback();
  return false;
 }
 return true;
}