NHibernate - access the ID of an associated object without lazy loading the whole object

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=tru

Answer 1

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 2

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 3

You can use GetIdentifier method of Nhibernate session :

session.GetIdentifier(obj);