create table PARAMETER (
  ID                    INTEGER         NOT NULL,
  NAME                  VARCHAR2 (12)   NOT NULL,
  ROW_VERSION           INTEGER         NOT NULL,

  // ...

  constraint PARAMETER_PK primary key (ID)
);


create table PARAMETER_SET (
  ID                    INTEGER         NOT NULL, 
  // -- KEY via INDEX --
  REQUEST_ID              VARCHAR2 (10)   NOT NULL,
  TRANSACTION           VARCHAR2 (10)    NOT NULL,
  PARAMETER_ID          INTEGER         NOT NULL,
  // ---------
  ROW_VERSION           INTEGER         NOT NULL,
 
  // ...
 
  constraint PARAMETER_SET_PK primary key (ID)
);

create table REQUEST (
  ID                    VARCHAR2 (10)   NOT NULL,
  ROW_VERSION           INTEGER         NOT NULL,
 
  // ...
 
  constraint REQUEST_PK primary key (ID)
);

// a unique index on the real key for ParameterSet
create unique index PARAMETER_SET_UI1 on PARAMETER_SET (
    REQUEST_ID,
    TRANSACTION,
    PARAMETER_ID
);

<class name="Request, Domain"
         table="REQUEST" lazy="false">

    <id name="ID" column="ID">
      <generator class="native">
        <param name="sequence">REQUEST_SEQ</param>
      </generator>
    </id>   

    <version name="RowVersion" column="ROW_VERSION" type="Int32" unsaved-value="0" />

    <property name="Name"             column="NAME" />

    <bag name   ="ParameterSets"
         inverse ="true"
         cascade ="all-delete-orphan"
         lazy    ="false">
      <key column="REQUEST_ID"/>
      <one-to-many class="ParameterSet, Domain"/>
    </bag>
</class>



<class name="ParameterSet, Domain"
         table="PARAMETER_SET" lazy="false">

    <id name="ID" column="ID">
      <generator class="native">
        <param name="sequence">PARAM_SET_SEQ</param>
      </generator>
    </id>
   
    <version name="RowVersion" column="ROW_VERSION" type="Int32" unsaved-value="0" />

    <property name="Transaction" column="TRANSACTION">
      <column name="TRANSACTION" unique-key="TEST" index="PARAMETER_SET_UI1"/>
    </property>

    <property name="Target"         column="TARGET" />

    <many-to-one name="Request" column="REQUEST_ID"
              class="Request, Domain"
              not-null="false"
              lazy="false"
              insert="true" update="false">
      <column name="REQUEST_ID" unique-key="TEST" index="PARAMETER_SET_UI1"/>
    </many-to-one>

    <many-to-one name="Parameter" column="PARAMETER_ID"
              class="Parameter, Domain"
              not-null="false"
              lazy="false"
              insert="true" update="false">
      <column name="PARAMETER_ID" unique-key="TEST" index="PARAMETER_SET_UI1"/>
    </many-to-one>

</class>



<class name="Parameter, Domain" table="PARAMETER">

    <id name="ID" column="ID">
      <generator class="assigned" />
    </id>

    <version name="RowVersion" column="ROW_VERSION" type="Int32" unsaved-value="0" />

    <property name="Name"             column="NAME" />
</class>

Request request = // Load a request

// The ParameterSets of the Reuqest contains an object with
//    Transaction = "T1"
//    Reuqest      = request
//    Parameter   = Parameter (PARAM1)
request.ParameterSets.Clear();

ParameterSet ps = new ParameterSet();
// A part of the data of the new ParameterSet is set to:
//    Transaction = "T1"
//    Reuqest      = request
//    Parameter   = Parameter (PARAM1)

request.ParameterSets.Add(ps);

// The Error is thrown here:
HibernateSession.SaveOrUpdate(request);

// The problem is, on the Database must be an unique index on the columns Transaction, Request_ID and Parameter_ID to allow only one unique object with this combination.

Hibernate flusing: could not insert: [Domain.ParameterSet#2538][SQL: INSERT INTO PARAMETER_SET (ROW_VERSION, TRANSACTION, MODIFICATION, SEQUENCE_NUMBER, TARGET, LOWER_SPEC_LIMIT, UPPER_SPEC_LIMIT, STOP_FLAG, OPTIONAL_FLAG, REQUEST_ID, PARAMETER_ID, ID) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)]; SQL []; ORA-00001: Verstoß gegen Eindeutigkeit, Regel (DBXYZ.PARAMETER_SET_UI1)