Question

It seems that altough I have been using NHibernate for a while now, I still misunderstand some basic concepts of this ORM. Let’s say I have a class called “Blog” and I load a persisted instance like so:

using (var tx = Session.BeginTransaction())
{
    var myBlog = Session.Get(10);

    tx.Commit();
}

If I now change a property of this instance, NHibernate seems to automatically detect the unsaved changes and will produce an UPDATE on transaction-commit.

This causes, that the following statements do exactly the same:

using (var tx = Session.BeginTransaction())
{
    var myBlog = Session.Get(10);
    myBlog.Title = "Changed title";

    tx.Commit();
}

using (var tx = Session.BeginTransaction())
{
    var myBlog = Session.Get(10);

    myBlog.Title = "Changed title";
    Session.Update(myBlog); // why is this necessary?

    tx.Commit();
}

I don’t see any difference with NHProf. So why does the explicit Update-method exists and when should I use it?

Answer 1

Entities are not always connected with session. For example you could have webservice with method, that accepts some entity, and updates in db:

[WebMethod]
void UpdatePerson(int id, string name){
    using (var tx = Session.BeginTransaction(){
        var person = new Person(id, name);
        Session.Update(person);
        tx.Commit();
    }
}

This code executes update in database without issuing select.