fluent nhibernate - many-to-many relationship mapping on same entity

Question

I am having a problem trying to map out a many-to-many relationship , where both sides of the relationship reference the same entity. I am using Fluent NHibernate and NH3.1.

Basically, the scenario is this - I have a category, which can have multiple parents. Thus, a category has multiple other categories as parents, as well as multiple other categories as its children.

HasManyToMany(x => x.ParentCategories).AsBag().Table("parentcategorychildren").ParentKeyColumn("ChildID").ChildKeyColumn("ParentID").Cascade.SaveUpdate();
HasManyToMany(x => x.ChildrenCategories).AsBag().Table("parentcategorychildren").ParentKeyColumn("ParentID").ChildKeyColumn("ChildID").Inverse();

However, when I try to build the factory, I get the following error:

The relationship Category.ChildrenCategories to Category.ChildrenCategories has Inverse specified on both sides. Remove Inverse from one side of the relationship.

What I am finding strange is why is it mentioning 'Category.ChildrenCategories' to Category.ChildrenCategories, as opposed to ParentCategories?

Any help would be greatly appreciated !

I just created a bounty for this, because it's important enough to me. Please, I'm not interested in "you can't do this" as an answer.

Answer 1

This is most likely a FNH bug and it is most likely already fixed in the latest FNH source code. There is no problem when using FNH1.0 and NH2.1. Equivalent HBM mapping works well in FNH1.2 and NH3.1:

<bag name="ParentCategories" cascade="all" table="parentcategorychildren">
    <key column="ChildID" />
    <many-to-many column="ParentID" class="Category" />
</bag>

<bag name="ChildrenCategories" inverse="true" table="parentcategorychildren">
    <key column="ParentID" />
    <many-to-many column="ChildID" class="Category" />
</bag>

EDIT: After digging in FNH source code I found a workaround. Let's say, your configuration looks like this:

.Mappings(m => {
    m.FluentMappings.AddFromAssemblyOf<Category>();
})

The unlucky code can be suppressed by this configuration:

.Mappings(m => {
    var persistenceModel = new PersistenceModel();
    persistenceModel.AddMappingsFromAssembly(typeof(Category).Assembly);
    persistenceModel.ValidationEnabled = false; // this makes the trick
    m.UsePersistenceModel(persistenceModel);
})

Answer 2

This is an issue with Fluent NHibernate 2.1's validation/pairing of relationships. FNH pairs up relationships and then validates that only one side of the relationship has .Inverse() specified. Because both references (parent/child) are to the same class they are both candidate matches when pairing. In that case, FNH matches on name similarity. Consequently, they each get paired with themselves rather than each other. So then placing .Inverse() on either one triggers the validation (both sides of the pair are the same relationship which is inverse).

It should be possible to correct this using the OverrideBiDirectionalManyToManyPairing() method on the FluentMappingsContainer. In theory, that would allow you to explicitly pair the child and parent relationships. However, in FNH 2.1 there is a bug, and the override callback is never called. (The callback value is captured before it can be set by the method).

As a work around, you can disable all validation in FNH. There are only two validations. First, that both sides of a relationship don't have .Inverse(). Second, that an Id is mapped on each entity. The cleanest way I have found to disable validation is:

.Mappings(m => {
    var persistenceModel = new PersistenceModel() { ValidationEnabled = false };
    m.UsePersistenceModel(persistenceModel)
     .FluentMappings.AddFromAssemblyOf<Category>();
})

This approach allows you to disable validation but still use the full expressiveness of the FluentMappings configuration.