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Multi Table Entities in NHibernate
A while ago I posted about the ability to map n tables to a single entity in the Entity Framework. I didn't like it then, and I quoted from the Hibernate documentation that discourage this behavior:
I still believe that this statement is true, except... I just run into an issue with my model, I have a case where I am importing data from another database, and I need to add additional data to it. I could add additional columns to the primary table, but that would make the import process much more complex than I would like it to be. I would have liked to make it work by splitting the data by table, rather than by columns.
With that in mind, I headed to NHibernate's JIRA, and found this issue about the problem. Conveniently, a patch was supplied as well.
A big thanks for Karl Chu for making all the work of porting the functionality from Hibernate. I love Open Source.
At any rate, you can now map several tables into a single entity in NHibernate, you can get the full details here (the new tests), but let us walk through a simple one first.
Name and sex are defined in the Person table, but everything else is defined on the Address table, we map it like this:
<class name="Person">
<id name="Id" column="person_id" unsaved-value="0">
<generator class="native"/>
</id>
<property name="Name"/>
<property name="Sex"/>
<join table="address">
<key column="address_id"/>
<property name="Address"/>
<property name="Zip"/>
<property name="Country"/>
<property name="HomePhone"/>
<property name="BusinessPhone"/>
</join>
</class>
Obviously address_id is a FK to person_id (not the best names for them, come to think about it). Trying to load a person would cause this SQL query (reformatted):
SELECT
p.person_id,
p.Name,
p.Sex,
a.Address,
a.Zip,
a.Country,
a.HomePhone,
a.BusinessPhone
FROM dbo.Person p inner join dbo.Address a
on p.person_id=a.address_id
There is quite a bit more that it can do (optional joins, etc), and you can check it out at the tests.
Note: this is on NHibernate trunk, so it won't be in the 1.2 release, which is currently in feature-freeze.
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