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Free NHibernate mapping for category / subcategory tree

I have the following model class for my Category object :

public virtual int Id { get; set; }
public virtual string Name { get; set; }
public virtual Category ParentCategory { get; set; }

public virtual IList<Category> SubCategories { get; set; }

This is my free nhibernate mapping class for ' Category ':

Id(x => x.Id).GeneratedBy.Native();

Map(x => x.Name);
References(x => x.ParentCategory).Column("ParentCategoryId");

// ** THE BELOW MAPPING IS WHAT I'M UNSURE ABOUT **
HasMany(x => x.SubCategories).Where(x => x.Id == x.ParentCategory.Id);

My database, to which this refers, consists of several "categories", some of which are at the root level (and have the value ParentCategoryId = NULL), and all the rest are subcategories, which can only be at 1 level of depth or can be 3 , 4,5 levels (recursive parents return to the root / parent CategoryId.

An example of the relationship between rows / records:

Cars (Id = 1 - ParentCategoryId = NULL)
Cars (Id = 1) > Hatchback (Id = 2 - ParentCategoryId = 1)
Cars (Id = 1) > Hatchback (Id = 2) > Ford (Id = 3 - ParentCategoryId = 2)

Motorcycles (Id = 4 - ParentCategoryId = NULL)
Motorcycles (Id = 4) > Scooters (Id = 5 - ParentCategoryId = 4)

SubCategories ' ParentCategoryId (Id), . HasMany, , .

+3
1

, , HasMany :

HasMany(x => x.SubCategories)
.Cascade.AllDeleteOrphan()
.KeyColumn("ParentCategoryId")
.Where(x => x.ParentCategory.Id == x.Id)
+3

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