Hello, I’m working with NHibernate and VB.Net. I need to load a many-to-one property in a “lazy” way and I don’t know how I can develop it.
I was looking for a code example using a proxy class but I couldn’t find it.
Could anybody share an example?

as far as I know, you can't really define a m-1 relationship as lasy, however you can prevent it from being loaded by using "outer-join=false" setting.

One thing you could try would be to declare the lazy attribute in the class of the m-1:



if that doesn't work, i know



will prevent the class from being loaded. However, you sort of rely on that class being in the second-level cache if you need to reference it...

-devon

Hmmm... Shouldn't many-to-one be lazy if the class is defined as lazy? Let's say you have Person and Title:



Title is defined as lazy="true" in class element, so a <many-to-one> from Person will be lazy... No? That's how it works in my code anyway...

Hi, thanks for your reply.
I’ve tried to do it but I have some trouble.
I defined my classes as follow

<class name="Provincia" table="Provincias">
<id name="Id" type="Int32" unsaved-value="0">
<column name="idProvincia" sql-type="int" not-null="true" unique="true" />
<generator class="native" />
</id>
<property name="Descripcion" type="String">
<column name="Descripcion" length="60" sql-type="varchar" not-null="true"/>
</property>
<many-to-one name="Pais" class="Pais">
<column name="IdPais" sql-type="int" not-null="true"/>
</many-to-one>
</class>


<class name="Pais" table="Paises" lazy="true" >
<id name="Id" type="Int32" unsaved-value="0">
<column name="IdPais" sql-type="int" not-null="true" unique="true" />
<generator class="native" />
</id>
<property name="Descripcion" type="String">
<column name="Descripcion" length="60" sql-type="varchar" not-null="true"/>
</property>
<bag name="Provincias" inverse="true" lazy="true" cascade="none">
<key column="idProvincia"/>
<one-to-many class="Provincia"/>
</bag>
</class>

When I execute the following instruction
mySession.Find("from IAMCReportes.Common.Provincia as Provincia")
Only a select on Provincias table is done, that's correct. But when I try to access property Pais it is never loaded.
How can I load the Pais when it is required?

That really depends on how you are building your application. Basically, you have to understand what is going on in the background with regard to lazy initialization. Lazy collections can be instantiated on the fly if you still have the original session open, but that isn't always the best method. In most instances, the preferred method is to build the entire object graph necessary before it is forwarded to the View ( as in the MVC pattern).

If you follow the recommendations of the Hibernate team, all associations should be mapped lazy and you should build specific HQL queries to return specific instances of object graphs necessary for a particular purpose. In your case, you might write an HQL like so:



An alternative would be to use the session.Lock() method and then initialze the lazily loaded attribute (property).

In my opinion, you shouldn't be lazily loading the Pais. Paises don't change much (even with world politics being what they are). I'd feel pretty safe allowing Pais to be eagerly fetched on the m-1 association and instead enable a "read-only" cache on the Pais object. It isn't going to change much and if it does, it will almost certinaly require a restart of the app. This way, NH will load the Pais from the cache and not the database. You could do this with a simple change to the mapping file:



and the addition of the cache notation to the Pais mapping:

Hi again, the relation between Pais and Provincia is just an example, my specific problem is that I have some objects that have M-1 relation with others objects and these also have M-1 relation with others, these relations cause too many accesses to de DataBase. These objects change a lot so I can’t use the cache.

I’m trying to load these M-1 relations in a lazy way, I changed the mappings files to use Lazy=”true”, and with this change I can load Provincias and its Pais doesn’t load, but when I want to see its Pais’s description property, how can I load this object?

sorry martin,

i guess, if you do not want to add a specific method to build your object graph for each use case, then you should add a generic method to your generic DAO (assuming you have one) that loads a property or object appropriately:



-devon

Thank Devon, It’s my first experience on NHibernate and I’m a little confused.
Where do I call the InstantiateLazyProperty function? And when?
Could you send me some code example?

Thank you very much.

Martin

martinvicente: did you declare all the public properties and methods of your entities as virtual (overridable)?

I tested it and it worked fine.
Thanks to devonl, Balagan and sergey for your help.
Martin