How to map many-to-many relations with additional properties

breskeby · **Posted:** Mon Jun 19, 2006 7:55 am

I want to map a many-to-many relation, which is saved in a relation. the problem is, that this relation has additional properties, so i can´t use the many-to-many syntax i guess. but i have no clue how to map that. anybody a good idea?

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tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

You can do this using composite elements and many-to-one (refdocs 8.2 and 23.3). Or you can map the join table as a separate entity (refdocs 7.5.1, one-to-many/many-to-one). Or, if your join table has only a single additional property, being a sequence, then you can use a list with many-to-many. This situation crops up quite a lot, and it's very handy that hibernate handles it so cleanly.

breskeby · **Posted:** Tue Jun 20, 2006 5:36 am

my mapping file looks now as the following:

Code:

<?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 package="orm">
   <class name="ObjectItem" table="OBJ_ITEM">
      <meta attribute="class-description">Represents an Object Item</meta>
      <id name="id" column="obj_item_id" type="java.lang.Long">
         <generator class="native"/>
      </id>
      <property name="categoryCode" column="cat_code" type="string" length="6" not-null="true"/>
      <property name="name" column="name" type="string" length="100" not-null="true"/>
      <property name="alternateIdentificationText" column="altn_identific_txt" type="string" length="255"/>
      <property name="ownerId" column="owner_id" type="long" not-null="true"/>
      <property name="updateSeqNo" column="update_seqnr" type="long" not-null="true"/>
      <list name="capabilities" table="OBJ_ITEM_CAPAB" cascade="all">
         <meta attribute="use-in-tostring">true</meta>
         <key column="obj_item_id"/>         
         <index column="obj_item_capab_ix" />
         <composite-element class="ObjectItemCapability">
            <many-to-one name="capability" class="Capability" cascade="all">
               <meta attribute="use-in-tostring">true</meta>
               <column name="capab_id"/>
            </many-to-one>            
            <property name="missionPrimacyCode" column="msn_primacy_code" type="string" length="6"/>
            <property name="reportingDataId" column="rptd_id" type="long"/>
            <property name="ownerId" column="owner_id" type="long" not-null="true"/>
            <property name="updateSeqNo" column="update_seqnr" type="long" not-null="true"/>
         </composite-element>
      </list>
      <list name="locations" table="OBJ_ITEM_LOC" cascade="all">
         <meta attribute="use-in-tostring">true</meta>
         <key column="obj_item_id"/>         
         <index column="obj_item_loc_ix" />
         <composite-element class="ObjectItemLocation">      
            <many-to-one name="location" column="loc_id" class="Location" cascade="all">
               <meta attribute="use-in-tostring">true</meta>
            </many-to-one>            
            <property name="accuracyQuantity" column="acc_qty" type="double" not-null="false"/>
            <property name="bearingAngle" column="brng_angle" type="double" not-null="false"/>
            <property name="bearingAccuracyAngle" column="brng_acc_angle" type="double" not-null="false"/>
            <property name="speedRate" column="speed_rate" type="double" not-null="false"/>
            <property name="speedAccuracyRate" column="speed_acc_rate" type="double" not-null="false"/>         
            <property name="useCatCode" column="use_cat_code" type="string" length="6" not-null="true"/>
            <property name="reportingDataId" column="rptd_id" type="long"/>
            <property name="ownerId" column="owner_id" type="long" not-null="true"/>
            <property name="updateSeqNo" column="update_seqnr" type="long" not-null="true"/>
         </composite-element>
      </list>
   </class>
</hibernate-mapping>

the only wish i have, is that the obj_item_capab_ix and the obj_item_loc_ix will be generated using the native generator. how to do that? i tried it somehow, but it doesn´t work!

tenwit · **Joined:** Thu Dec 23, 2004 9:08 pm **Posts:** 2008

Those two columns, being defined as index (same as list-index, as I found out two days ago), will be the index of the composite element within the list. If you use base="1", 1 will be added to the java list's index in order to come up with the database id value. I think that's pretty much what the native generator does, isn't it? The only thing that's potentially different is if the native generator is falling back to identity, then it won't work, because identity means that the number is chosen by the DBMS, whereas index is assigned by java/hibernate.

The easy solution (assuming that the obj_item_capab_ix and obj_item_loc_ix columns are currently sequences/identities) is to make those columns simple ints, and drop the DB sequence/identity requirement. Hibernate will implicitly guarantee the same effect for you, because (obviously) you can't have two items at index 1 in a list. The downside is that a particular row might have id 2 when it's loaded, but if row 1 is deleted, row 2 will be saved as row 1 afterwards. But hibernate looks after all of that for you, so usually you don't have to worry about it.

If that's not suitable for you, I believe that you'll have to switch to using the one-to-many/many-to-one style mapping (refdocs section 8.2), in which ObjectItemCapability and ObjectItemLocation are mapped as their own classes, rather than as composite elements.