Question

I have two associated business objects - A and B. the association is (A->B)many-to-one, with B.Id a foreign key in A (so A has A.B_id in the DB).

I m using lazy=true and solved most of my problems, however in A s ToString I want to print also A.B.Id, which I should have without further trips to the DB. but accessing A.B activates the proxy, and since this isn t in the context of an open session, throws an exception.

one easy but ugly solution would be to have A.B_id property. but that s part of the stuff we were trying to avoid in the first place. any "organic" way to do this? :) thanks!

UPDATE: just read about caching and Session.Get vs. Session.Load. before I only new that one throws an exception if the object doesn t exist (Session.Load), and the other returns a null object (Session.Get). after reading about caching here, it s clear that Session.Load returns a proxy to the object, and only lazily fetches it when a property other than the ID is accessed, which is very much like what I need from associations! for now I added the separate object ids (added B_Id to A so I can access it as A.B_Id instead of using A.B.Id)

Answer 1

For the exact same reason I have used explicit A.B-ID properties for all my many-to-one relationships. I do not see this as a quick and dirty solution as it provides a solution to this problem and also lot of flexibility is the saving-updating area i.e. I do not need to fetch from the database the B object just to assign it to A in order to create the association when I have the B_ID in a query string or somewhere else.

My mapping files useually look like this:

<property name="CreatorID" column="CreatorID" type="Int32" not-null="true" />
<many-to-one name="Creator" column="CreatorID" class="SystemUser" insert="false" update="false" cascade="none" />

As you can see one of the 2 properties has to be read only to avoid having NHibernate sending 2 times this column to the database when inserts or updatas are happening. The above makes as read only (by using the insert="false" update="false" attributes) the many-to-one but you can instead have as read only the CreatorID property if you like.

Having only the many-to-one you do not have a property in your entity class A to hold the B.ID value. The only way to get it is by accessing the B object which will trigger the proxy and it will fire a query to the database (if it is not loaded in the session already).

I will be happy to hear any other option that provides a solution and offers the same kind of flexibility.

Answer 2

If you are using NHibernate 3.2 or later, you could use the following code to get the id of associated object without another roundtrip to database to load the whole object:

using NHibernate.Proxy;
...
object id = null;
if (obj.IsProxy()) // obj is the object you want to get its identifier.
{
    var proxy = obj as INHibernateProxy;
    if (proxy != null)
    {
        var li = proxy.HibernateLazyInitializer;
        if (li != null) 
            id = li.Identifier;
    }
}

Answer 3

You can use GetIdentifier method of Nhibernate session :

session.GetIdentifier(obj);

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