Handling foreign keys in code

cremor · **Joined:** Tue Jul 29, 2008 3:30 am **Posts:** 74

Hi,

I'm new to NHibernate and have a litte problem with my foreign keys in code.
I have mapped the foreign keys to object references in C#, which is working great. But I wonder what is the best way to set that reference if you only know the ID of the referenced object and don't have a object instance.

One way would be to load the object instance with the ID from the database. But that causes a unneeded select statement, because I don't really want to know anything about the referenced object except it's ID.

Then I tried to add a second property to my business objects which represent my foreign keys. So I had the reference to the foreign object and the foreign key as a int property. But I couldn't find a way to say NHibernate that these two properties map to the same column. And because of that inserting and updating didn't work. Have I missed something here or is that a wrong way to do it?

Right now I just create a new object myself and set it's ID like this:

Code:

child.Parent = new Parent { ID = parentID };

Is there any better way to do it?

NHibernate version: 1.2.1.GA
.NET 3.5
ASP.NET Website, one NHibernate-Session per Request

wolli · **Posted:** Wed Jul 30, 2008 7:01 am

You can try something like this:

Code:

class Child {
...
int ParentId { 
    get { return parent != null ? parent.Id : parentId; set { parentId = value; }

Parent Parent { get {...}; set {...} } 

}

And a mapping like this:

Code:

<property name="ParentId" column="parentid" />
<many-to-one name="Parent" column="parentid" insert="false" update="false"/>

cremor · **Joined:** Tue Jul 29, 2008 3:30 am **Posts:** 74

Thanks for the idea, but that won't work if I used the Parent-property (to get the name of the parent for example) because then the parent-field won't be null.

Clearing the parent-field in the setter of the ParentID-property would be an option, but again only until the next property of the Parent-property is requested.

Which leads me to the next problem: If I set the ParentID-property to a new ID and then request a property of the Parent (without first updating the database), the old Parent will be returned.

Any ideas?

cremor · **Joined:** Tue Jul 29, 2008 3:30 am **Posts:** 74

I found the solution for this. Instead of ISession.Get() you can use ISession.Load(). That method only returns a proxy object, so no database query is executed until someone really tries to access a property of that object.

Don't know why I didn't found this earlier :-(
It's also well documented:

Quote:

If the class is mapped with a proxy, Load() returns an object that is an uninitialized proxy and does not actually hit the database until you invoke a method of the object. This behaviour is very useful if you wish to create an association to an object without actually loading it from the database.