I have class hierarchy like the following.
Fruit
--Apple
----Red Delicious Apple
----Granny Smith
--Orange
--Strawberry

...etc

The db is structured as follows:
t_fruit
FruitKey int
FruitType int
FruitName varchar(255)

t_apple
FruitKey int
FruitType int

t_apple_grannysmith
FruitKey int
FruitType int

t_orange
FruitKey int
FruitType int

I would like to either modify NHibernate to use a discriminator with a joined-subclass or take a where clause as the parameter of the joined-subclass. Because constraints are used on the subclass FruitType columns a where clause would allow the db to execute much quicker by eliminating the multiple joins created in the select statements.

I am brand new to NHibernate, I am trying migrate a roll-your-own DAL into NHibernate and would love to see this type of feature supported.

Any thoughts from experts on where to start or if this is even possible?

According to described table structure you don't need <joined-subclass> at all: all of derived tables you've shown contains only id and type. If it really looks like this, then <subclass> will be enough.

On the other hand, if the given structure is just a sample and there are additional fields in joined tables, then you will anyway need joins to fetch all object's data from the database.

And the last: if you belive that your approach will help queries to run faster, you can check it first by creating database views and mapping your classes to those views instead of tables.

For sake of brevity I withheld all the column props. An Orange needs to be joined to the Fruit table so it can load all the fruit properties or am I missing something?


This would be a good exercise I will try and test this to get some real numbers.

Everything in the system is a subclass of Fruit, technically an object called Item, each concrete subclass has a table with the additional properties related to the subclass. The reason every table also has an ItemTypeKey field was to enforce constraints on the items being inserted as well as provide better performance for generic selects. The old DAL was also able to use the ItemTypeKey as a discriminator to know what object to instantiate.

I was looking at Hibernate 3.0 and it seems they have the functionality I require. Looks like I need to wait until NHibernate gets an update.

Is there a roadmap on getting a comparable NHibernate 3.0 release?

I think i understand you: we also have "object type" field in a base table for joined-subclass hierarchy, and i also would like joined-subclasses to support discriminators. But it would help performance in one case only - then you use base class in a query. In this case NH must join all the subtables in one select (usin left joins) and then select object class according to empty parts of the resultset. If you specify "leaf" class in a query, it works quite optimal already.

You see, it's not a matter of where clause, it's more complex question - how could you load instances of several possible types in one select w/o joining their tables alltogether?