public class Product : BaseEntity
    {
        public Product()
        {
            Categories = new List<Category>();
            Units = new List<Unit>();
        }
        
        public virtual string Name { get; set; }
        public virtual IList<Category> Categories { get; set; }
        public virtual IList<Unit> Units { get; set; }
        public virtual Tax Tax { get; set; }
        public override string ToString() { return Name; }
    }

public class ProductMap : ClassMap<Product>
    {
        public ProductMap()
        {
            Id(c => c.id);
            Map(c => c.Name).Column("Name").Not.Nullable();
            References(l => l.Tax)
            .Column("idTax")
            .Not.Nullable();

            ////многие ко многим, связь через таблицу
            HasManyToMany(i => i.Categories)
                .Cascade
                .All()
                .Inverse()
                .ParentKeyColumn("idProduct")
                .ChildKeyColumn("idCategory")
                .Table("ProductCategories");

            HasManyToMany(i => i.Units)
                .Cascade
                .All()
                .Inverse()
                .ParentKeyColumn("idProduct")
                .ChildKeyColumn("idUnit")
                .Table("ProductUnits");

            Table("Product");
        }
    }

var obj = session.Query<Product>()
                    .Fetch(t => t.Tax)
                    .FetchMany(u => u.Units)
                    .FetchMany(c => c.Categories).ThenFetch(p => p.Parent)
                    .Where(x => x.id == 1).SingleOrDefault();