NHibernate, polymorphic associations and ghost objects
One of the more interesting points of my posts about Entity Framework & NHibernate is the discovery of things that Entity Framework can do that NHibernate cannot. In fact, if you’ll read the posts, instead of the comments, you can see that this is precisely what I asked, but people didn’t really read the text.
I wanted to dedicate this post to ghost objects, and how NHibernate deals with them.
Before we start, let me explain what ghost objects are. Let us say that you have a many to one polymorphic association, such as the one represented as Comment.Post.
A post may be either a Post or an Article, and since NHibernate by default will lazy load the association, NHibernate will generate a proxy object (also called a ghost object). That, in turn, result in several common issues: Leaking this and the inability to cast to the proper type are the most common ones.
In practice, this is something that you would generally run into when you are violating the Liskov Substitution Principle, so my general recommendation is to just fix your design.
Nevertheless, since the question pop up occasionally, I thought that I might write a bit more details on how to resolve this. Basically, the main issue is that at the point in time where we are loading the Comment entity, we don’t have enough information to know what the actual entity type is. The simplest way to work around this issue is to tell NHibernate to load the associated entity as part of the parent entity load.
In the case of the comment, we can do it like this:
<many-to-one name="Post" lazy="false" outer-join="true" column="PostId"/>
The lazy=”false” tell NHibernate to load the association eagerly, while the outer-join will add a join to load it in a single query. One thing to note, however, is that (by design) HQL queries will ignore any hints in the mapping, so you would have to specify join fetch explicitly in the mapping, otherwise it would generate a separate query for that.
Since we eagerly load the associated entity, and we know its type, we don’t have to deal with any proxies, and can avoid the ghost objects problem completely.