NHibernate Get vs Load and sub types

Here’s some interesting NHibernate behaviour that’s been tripping me up. Say we have a base class ‘Content’ and a sub class ‘Menu’, the following tests pass:

[Test]
public void Get_returns_a_proxy_of_the_correct_type()
{
   InSession(session =>
   {
       var content = session.Get<Content>(menu1Id);
       var menu = content as Menu;
       menu.ShouldNotBeNull("menu is null");
   });
}

[Test]
public void Load_does_not_get_return_a_proxy_of_the_correct_type()
{
   InSession(session =>
   {
       var content = session.Load<Content>(menu1Id);
       var menu = content as Menu;
       menu.ShouldBeNull();
   });
}

That’s right, don’t expect ‘as’ to work on lazily loaded entities.

Get eagerly loads, so it runs some SQL against the database:

SELECT
   content0_.ContentId as ContentId15_0_,
   content0_.Name as Name15_0_,
   content0_.Position as Position15_0_,
   content0_.IsActive as IsActive15_0_,
   content0_.UrlName as UrlName15_0_,
   content0_.ParentContentId as ParentCo7_15_0_,
   content0_.ContentTypeId as ContentT8_15_0_,
   content0_.Controller as Controller15_0_,
   content0_.Action as Action15_0_,
   content0_.Text as Text15_0_,
   content0_.ContentType as ContentT2_15_0_
FROM "Content" content0_
WHERE content0_.ContentId=@p0;@p0 = 1 [Type: Int32 (0)]

This includes the discriminator column, ContentType, so NHibernate knows that this item should be created as a menu.

Load, lazy loads, so for this example no SQL is run against the database. The only thing NHibernate knows about this entity is that we’ve asked for ‘Content’ via the type parameter of the Load method, so it returns a proxy class that inherits from Content. When we use the ‘as’ operator to cast it to a menu it fails (returns null).

Read more: Code rant

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