I've quite recently discovered NHibernate, so forgive me if this is a beginners question, but I can't find the answer anywhere on Google.

I have three tables, two entity tables and one relation table.


These are modeled in C# like this.


That displays the relation in code that I want.
I want to be able to select BlogPosts in a certain Category or get all Categories that a certain BlogPost is in.

Category is mapped as an usual entity with Unique key on the Name property.
BlogPost is mapped as usual, but the Categories property is a <set> with <many-to-many> like this.


The problem is the following, when doing inserts (I add new Category to the Categories collection) I get exceptions that there already is a Category with that name (Duplicate unique key). How can I tell NHibernate to check before doing insert and if there's a value, it should use that Id instead of trying to insert? This is really bothering me since this isn't a very complex technique. It's just normal normalization and common sense.

I appreciate the help.

Kind regards, Kim Johansson

I think I've solved it. I use the same category object for each BlogPost I insert in the test, and then later on I just go through the Categories and check which is already in the database manually.

It may not be nice, but it works.

You could get yourself a map/dictionary of all categories.

Something like:



where the String Parameter is the Category Name.

Mapping file would look something like the following:



When you're inserting a new Blogpost you can use this Dictionary to look up if there's already an entry with your unique Category Name.

If that doesn't fit your needs maybe you can have a look at Lifecycle Callbacks (chapter 4.4 in the nhibernate manual).

Another idea might be to make the Category Name the primary key (though this is not very nice).