Question

I have an entity that maps to a table called Rule. The table for this entity has an FK to another Table called Category. I’m trying to figure out how to pull in a property from Category in my Rule entity. I’m pretty sure I want to use a join in my entity mapping, but I can’t figure out how to configure it so that it works. Here is my mapping:

Join("Category", x => 
{
    x.Map(i => i.CategoryName, "Name");
    x.KeyColumn("CategoryId");
    x.Inverse();
});

Here is the SQL that it’s generating…

SELECT ...
FROM Rule rules0_ left outer join Category rules0_1_ on rules0_.Id=rules0_1_.CategoryId
WHERE ...

Here is the SQL that I want.

SELECT ...
FROM Rule rules0_ left outer join Category rules0_1_ on rules0_.CategoryId=rules0_1_.Id
WHERE ...

I can’t seem to find anything on the JoinPart that will let me do this. Subselect looks promising from the little bit of documentation I’ve found, but I can’t find any examples of how to use it. Any advice on this problem would be much appreciated. Thanks!

Answer 1

“Join” is poorly named. a “join” in an NHibernate mapping implies a zero-to-one relationship based on a relation of the primary keys of the two tables. You would use a join if, for instance, you had a User table and a UserAdditionalInfo table, with zero or one record per User. The UserAdditionalInfo table would likely reference the PK from User as both a foreign key and its own primary key. This type of thing is common when a DBA has to religiously maintain a schema for a legacy app, but a newer app needs new fields for the same conceptual record.

What you actually need in your situation is a References relationship, where a record has a foreign key relationship to zero or one other records. You’d set it up fluently like so:

References(x=>Category)
    .Column("CategoryId")
    .Inverse()
    .Cascade.None();

The problem with this is that Category must now be mapped; it is a separate entity which is now related to yours. Your options are to live with this model, to “flatten” it by making the entity reference private, changing the mapping to access the entity as such, and coding “pass-throughs” to the properties you want public, or by using a code tool like AutoMapper to project this deep domain model into a flat DTO at runtime for general use. They all have pros and cons.