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torpid
 Post subject: many-to-many AND many-to-one for same association?
PostPosted: Fri Oct 21, 2005 6:07 am 
Newbie

Joined: Fri Oct 21, 2005 4:49 am
Posts: 13
Hibernate version:
3.0.5
Name and version of the database you are using:
mySQL 4.1.14

I've got my application working fine apart from one issue.

I have a many to many association between A and B. In the mapping files these are explicitly called many to many.

However, I need to store some extra information about the association. There is an extra column in the link table and the A and B thus have an extra many to one association to the link class AB.
The link table has combo of both foreign keys of A and B as the PK.
The mapping file for AB generated by middlegen includes a composite key but instead declares the only non-key property (extraInfo) in the key.
Code:
MIDDLEGEN GENERATED VERSION
<class name="AB" table="AB">
   <composite-id>
        <key-property name="extraInfo" type="String">
   </composite-id>
<many-to-one name="A" class="A" not-null="true">
    <column name = "AID">
</many-to-one>
<many-to-one name="B" class="B" not-null="true">
    <column name = "BID">
</many-to-one>
</class>

<class name="A" table="A">
  <id name="A" column="A" type="integer">
<set name="Bs" lazy="true" cascade="save-update" table="AB" >
    <key><column name="AID"></key>
    <many-to-many class="B"><column name="BID"></many-to-many>
</set>
<set name="AB" lazy="true" cascade="save-update" inverse="true">
   <key><column name="AID"></key> <!-- i guess this should be composite too-->
   <one-to-many class="AB"/>
</set>
</class>

similarly for B but the many-tomany to A is mapped inverse=true

If I try to correct this, (using <key-many-to-one> or putting AID and BID in <composite-id>) the application won't even start.
Code:
______              ______
  A   |*----------*| B    |
PK:AID|            |PK:BID|
______|            |______|
    1               1
       *___________*
       | AB        |
       |PK: AID,BID|
       |extraInfo  |
       -------------

The problem is that when I assign an A to a B (or vice versa) a line is correctly put into the AB table but always the default value of extraInfo is saved. I have tried all sorts of different ways of changing this value, none of which work. What is the correct way to do this? Is it just a mapping issue or is there some java code that I haven't tried yet?

Thanks for your help[/code]
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ashneyde
 Post subject:
PostPosted: Fri Oct 21, 2005 8:52 am 
Beginner
Beginner

Joined: Tue Nov 25, 2003 3:33 pm
Posts: 35
yeah but how do you want hibernate to guess what your extraInfo is? There is nothing that will provide it with the extraInfo, so I guess the simplest way to do it is to introduce an extra object AB. Then your associations will look like this:

A one-to-many AB
B one-to-many AB

and

AB many-to-one A
AB many-to-one B
plus the extraInfo field to map

Note that all of your associations are now much simpler than many-to-many :-) Of course now you have to deal with an extra object :-(

HTH,
Alex.
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torpid
 Post subject: there is an AB object
PostPosted: Fri Oct 21, 2005 9:33 am 
Newbie

Joined: Fri Oct 21, 2005 4:49 am
Posts: 13
But there already is an AB object with the extraInfo field. The problem is the mapping I think. That can't be right as middlegen generated it. But Hibernate just gets stuck "processing foreign key constraints" on startup if I tell it what AB's actual PKs are.

Using middlegen's hbm file, causes a warning on startup along lines of cannot validate since can't find class AB. Even though it previously said it already mapped it...

I guess there is an issue because A and B have 2 associations to each other. One many to many which works beautifully AND a one-to-many with link table...

Thanks for the help
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ashneyde
 Post subject:
PostPosted: Fri Oct 21, 2005 11:26 am 
Beginner
Beginner

Joined: Tue Nov 25, 2003 3:33 pm
Posts: 35
yes, sorry what was I looking at :-)

I am not sure what middlegen is doing but here how my mapping for a situation like this usually looks:

Code:
<class name="AB" table="AB">
         <id
            name="id"
            column="id"
            type="long">
            <generator class="native" />
        </id>
        <many-to-one
            name="a"
            class="A"
            cascade="none"
            outer-join="auto"
            update="true"
            insert="true"
            column="aid"
        />
        <many-to-one
            name="b"
            class="B"
            cascade="none"
            outer-join="auto"
            update="true"
            insert="true"
            column="bid"
        />

       <property name="extraInfo" />

</class>


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jd001982
 Post subject: Re: many-to-many AND many-to-one for same association?
PostPosted: Fri Oct 21, 2005 9:50 pm 
Beginner
Beginner

Joined: Tue Oct 18, 2005 3:57 pm
Posts: 48
Location: Los Angeles, CA
torpid wrote:
I have a many to many association between A and B. In the mapping files these are explicitly called many to many.

However, I need to store some extra information about the association. There is an extra column in the link table and the A and B thus have an extra many to one association to the link class AB.

Try using this in you mapping file:
Code:
<set name="ab" table="AB">
  <key column="AID"/>
  <composite-element class="AB">
    <parent name="a"/>
    <many-to-one name="b"
                 class="B"
                 column="BID"
                 not-null="true"/>
    <property name="extraInfo" column="EXTRA_INFO" type="string" not-null="true"/>
  </composite-element>
</set>


And your classes will look something like this:
Code:
class A {
  private Set ab;
  ...
}

class AB {
  private A a;
  private B b;
  private String extraInfo;
  ...
}


Hope this helps (and don't forget to rate :D).

jd
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torpid
 Post subject:
PostPosted: Fri Nov 11, 2005 4:53 am 
Newbie

Joined: Fri Oct 21, 2005 4:49 am
Posts: 13
In the end, I left the mapping files as they were. The problem I had was that in the Java, I was calling
Code:
B b = new B();
a.getBs().add(b);
b.getAs().add(a);
AonB aob = new AonB(a, b);
AonB aob2 = session.get(aob, AonB.class);
aob2.setExtraInfo("extra");
session.save(aob2);


the code which works is just
Code:
session.save(new AonB(a,b,"extra"));


Hope this helps others
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torpid
 Post subject: Mapping needs to change
PostPosted: Fri Nov 11, 2005 7:22 am 
Newbie

Joined: Fri Oct 21, 2005 4:49 am
Posts: 13
A separate problem I had here
http://forum.hibernate.org/viewtopic.php?p=2270574#2270574

is down to this mapping issue. Any more suggestions?
Really stumped, any help would be greatly appreciated
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