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How to avoid unnecessary joins in NHibernate

Answer 1

A:

You must tell your config files that you want lazy-loading

Doh. So it turns out i wasn't correctly awake when i answered :)

I searched a bit about the subject and can point you towards this article by mookid (i guess he's here on SO too) thats confirms what Jamie Ides is saying which is that basically you may have a code smell here. Perhaps reconsidering the classes and their mapping could be a better idea.

samy 2010-10-05 23:05:07

...which I thought was the default so maybe we need to see the table mapping for "Entity"

Tahbaza 2010-10-05 23:18:45

I already have default-lazy="true" on my mapping file for this class. Should I do something more? I'll add the tests referenced in this article to see if they behave as they should.

David Montgomery 2010-10-05 23:27:20

@Tahbaza I thought the same, but since a weird bug in fluent-nhibernate i try to be exhaustive for diagnosticing :) @david do you try to gain access to your lazy-loading entities. If you do, that may be the reason it's trying to load the descendants..

samy 2010-10-05 23:43:13

Lazy loading has nothing to do with inheritance mapping

Paco 2010-10-06 10:56:06

@Paco Yeah, indeed. I read the question incorrectly and thought that the tables described a relationship mapping, not inheritance. After a good night sleep, the mistake is quite obvious :p

samy 2010-10-06 13:12:01

Answer 2

A:

Without making the join, NHibernate does not know to return you a User or Entity instance. You cannot avoid this join. When you save a user, you get back a user. If you don't want to "get back a user", you can save an entity instead of a user.

Paco 2010-10-06 09:06:10

Answer 3

+3 A:

Your mistake is that your database is poorly designed. You are using a table-per-subclass mapping strategy but you have applied it to many apparently unrelated classes. How can an Organization have a first or last name?

When you get an Entity, NHibernate has to join all the tables in the inheritance tree because it doesn't know what concrete type to return. If you get a User or an Organization it will only join to one other table as expected.

Is Entity an abstract class? You can't get concrete objects of type Entity because the abstract class can't be constructed.

Edited to add: After seeing the mapping files, I really think you should reconsider your design. This is not a good or reasonable use of inheritance. If you really want to continue with this model, I would suggest keeping the abstract base class but not mapping it. But I think this design has much deeper problems than that.

Jamie Ide 2010-10-06 11:50:26

I'm trying to implement a table-per-hierarchy mapping strategy, using joined tables to declare additional properties not found in the base class. Entity is not an abstract class; it is a concrete type; it has an abstract ancestor. Organizations and individuals are not types, they are differentiated by property values within the Entity class. So, your statement "If you get a User or an Organization it will only join to one other table as expected." is wrong - which is what prompted my initial question.

David Montgomery 2010-10-06 16:50:29

Does `Session.Get<User>(myIdValue)` join to multiple tables? Please show your mapping files.

Jamie Ide 2010-10-06 17:37:39

Hi Jamie, I added the existing mapping file to my original question

David Montgomery 2010-10-06 19:56:29

Answer 4

+1 A:

Polymorphism="explicit" should have been put into my class declaration. Polymorphism is implicit by default in NHibernate.

David Montgomery 2010-10-11 16:19:30

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