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jcavell
 Post subject: Mapping a Map with key and value from different tables
PostPosted: Wed Jun 13, 2007 11:14 am 
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Joined: Thu Jan 11, 2007 11:24 am
Posts: 7
I have a one-to-many relationship between a Brand and BrandItem. There is also a one-to-many relationship between an Item and BrandItem.

I want my Brand object to contain a Map of Item name (from the Item table) to BrandItem value (from the BrandItem table) - i.e. the key and value for the Map come from different tables.

Is this possible in Hibernate (without creating a view and creating the Map from this)?

Thanks
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jits_1998
 Post subject: Re:
PostPosted: Wed Jun 13, 2007 5:25 pm 
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Senior

Joined: Tue Jun 12, 2007 4:49 pm
Posts: 127
Location: India
This will get you a map of item and brandItem inside brand class.

Let me know if this will do the job.

Code:
<class name="Brand">
     <id name="id">
        <generator class="increment"/>
     </id>
     <map name="brandItems" table="brandItem">
        <key column="id"></key>
        <index-many-to-many class="Item" column="item" />
        <one-to-many class="BrandItem" />
     </map>
     <property name="name"/>
  </class>
 
  <class name="Item">
     <id name="id">
        <generator class="increment"/>
     </id>
     <set name="brandItems">
        <key column="item"/>
        <one-to-many class="BrandItem" />
     </set>
     <property name="name"/>
  </class>
 
  <class name="BrandItem">
     <id name="id">
        <generator class="increment"/>
     </id>
   <many-to-one name="brand" class="Brand" />
   <many-to-one name="item" class="Item" />
     
  </class>


Regards,
Jitendra
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jcavell
 Post subject:
PostPosted: Thu Jun 14, 2007 4:29 am 
Newbie

Joined: Thu Jan 11, 2007 11:24 am
Posts: 7
Thanks - that does help.

What this gives me is a Map, keyed on the stringifed Item class.

What I really want is it to be keyed on the value of a property from the Item class. I can achieve this by overriding toString() in the Item class, to give the value of the property I want; this isn't ideal.

Also, it performs two selects - one from BrandItem, and one from Item. I would prefer it to do a join.

Do you know if there is any way I can get round this two issues?

Thanks a lot

Jonny
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jits_1998
 Post subject: Re:
PostPosted: Thu Jun 14, 2007 6:49 am 
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Senior

Joined: Tue Jun 12, 2007 4:49 pm
Posts: 127
Location: India
Not sure what you mean by "stringified".

If you call brandItems.keySet() it will return you a set of Items. then you can do getName from the Item.

Quote:
Also, it performs two selects - one from BrandItem, and one from Item. I would prefer it to do a join.


Well hibernate is a very powerful tool, optimizations are possible at many levels, what I have provided is a simple solution that should work. Feel free to explore and find possible optimizations.

Regards,
Jitendra
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jcavell
 Post subject:
PostPosted: Thu Jun 14, 2007 7:21 am 
Newbie

Joined: Thu Jan 11, 2007 11:24 am
Posts: 7
By stringified, I meant the String representation of the object, as given by its toString() method.

Something I didn't mention is that I'm using 2nd level caching for the Map. I want to be able to access items keyed on the Item name, very frequently. I don't want to have to deal with a KeySet each time.

You mention optimisations, but do you know whether it's possible to configure Hibernate to put data into a Map using a join?

Thanks
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jits_1998
 Post subject: Re:
PostPosted: Thu Jun 14, 2007 8:41 am 
Senior
Senior

Joined: Tue Jun 12, 2007 4:49 pm
Posts: 127
Location: India
To optimize, there are attributes on map and index like formula, but I am not sure how to use them, you can experiment a bit and find out.

But to put it simply you can use toString approach, though I don't like it because the names are not unique and hence can lead to issues in future.

Regards,
Jitendra
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