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dircums
 Post subject: subclass with extended properties in another table
PostPosted: Mon Jun 18, 2007 7:46 am 
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I am trying to model a subclass using table-per-hierarchy.

My subclass also has extended properties which are held in another table.

Could someone give me some idea as to how to map this? I cannot find anyway to add a property from another table.

The nearest I can find is to add the properties of the other table as a component class, but this seems to me to be an un-intuitive way of doing it which is inconsistent with the rest of the object hierarchy.
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marcal
 Post subject:
PostPosted: Mon Jun 18, 2007 2:04 pm 
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The quick answer is: If you have a separate table holding properties for a given subclass, your strategy isn't really table-per-hierarchy. :)

Unfortunately, NH does not yet support mixed-mode inheritance. In general the way to "add a property from another table" is by using joined-subclass, but this can't be done in the context of a single-table hierarchy. From the docs (section 8.1): "NHibernate does not support mixing
<subclass> mappings and <joined-subclass> mappings inside the same <class> element."
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dircums
 Post subject:
PostPosted: Tue Jun 19, 2007 3:45 am 
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marcal wrote:
The quick answer is: If you have a separate table holding properties for a given subclass, your strategy isn't really table-per-hierarchy. :)


Well it is really... there are loads of common common/properties in the main table.. the seperate table just holds a couple columns with very specific properties and this is a link table to another entity specific to the subclass.

If there is no specific support for this I think I am going to have to kludge this by creating a specific association class
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dircums
 Post subject:
PostPosted: Tue Jun 19, 2007 4:39 am 
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Another idea!

Maybe I should mimic table-per-subclass by using views? Any comments?

I have just tried this and it seems to do the job OK. although this seems like a bit of a kludge, having to alter the database schema to get around a mapping problem.
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dircums
 Post subject:
PostPosted: Tue Jun 19, 2007 5:07 am 
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Boll*x

Can't do it this way... cannot save to database since it uses multiple tables.

Error: View or function 'DownloadPackages' is not updatable because the modification affects multiple base tables.

Here is my new mapping, which is against a view called download packages:


Code:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="Narrowstep.Entities" namespace="Narrowstep.Entities">
   <joined-subclass name="DownloadPackage" extends="Narrowstep.Entities.Vlip, Narrowstep.Entities" table="DownloadPackages">
      <key column="DownloadPackageID" />
      <property name="IsEnabled" column="IsEnabled" type="boolean" />
      <many-to-one name="PaymentRule" class="PaymentRule" column="PaymentRuleID" lazy="false" outer-join="true" />
   </joined-subclass>
   
</hibernate-mapping>


The original mapping was just a simple sublass element, but this did not allow me to use extended properties in the other table.
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dircums
 Post subject:
PostPosted: Tue Jun 19, 2007 5:07 am 
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.........................................................................................................
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karlchu
 Post subject:
PostPosted: Tue Jun 19, 2007 10:50 am 
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dircums,

You can download the latest snapshot, and then you can use the <join> mapping to achieve what you want.

http://jira.nhibernate.org/browse/NH-466

Karl
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marcal
 Post subject:
PostPosted: Tue Jun 19, 2007 10:37 pm 
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dircums wrote:
Can't do it this way... cannot save to database since it uses multiple tables.

Error: View or function 'DownloadPackages' is not updatable because the modification affects multiple base tables.

Here is my new mapping, which is against a view called download packages:


Darn, I'm ashamed that I didn't think of using views. It is still possible: if you are using SQL 2000/5 you should be able to build INSTEAD OF triggers to allow modifications to the underlying tables.

Quote:
Code:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="Narrowstep.Entities" namespace="Narrowstep.Entities">
   <joined-subclass name="DownloadPackage" extends="Narrowstep.Entities.Vlip, Narrowstep.Entities" table="DownloadPackages">
...   </joined-subclass>
   
</hibernate-mapping>


The original mapping was just a simple sublass element, but this did not allow me to use extended properties in the other table.


Interesting, I would have thought that you would use the view to pull the "DownloadPackages" columns into the main table and use <subclass> to map.
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