How to retrieve entities from a join using QueryOver in NHibernate - nhibernate

Question

I'm trying to return multiple entities from a QueryOver query. I'm doing this code in a plain text editor so there may be syntactic errors, but it should get the idea across.
public class Product
{
public virtual int ID { get; set; }
public virtual string ProductName { get; set; }
public virtual List<Category> Categories { get; set; }
public virtual List<Inventory> Inventories { get; set; }
...
}
public class Category
{
public virtual int ID { get; set; }
public virtual string CategoryName { get; set; }
public virtual string Style { get; set; }
public virtual Product Product { get; set; }
...
}
public class Inventory
{
public virtual int ID { get; set; }
public virtual List<Discount> Discounts { get; set; }
public virtual Product Product { get; set; }
public virtual bool InStock { get; set; }
...
}
public class Discount
{
public virtual int ID { get; set; }
public virtual Inventory Inventory { get; set; }
public virtual decimal DiscountAmount { get; set; }
...
}
Now my goal is to take a product ID and a couple other options to pull back a Category, Inventory, and DiscountAmount in a single query. I've gotten this to work using HQL with this query:
var query = session.CreateQuery("select category, inventory, discount.DiscountAmount"
+ " from Product product"
+ " join product.Categories category"
+ " join product.Inventories inventory"
+ " left join inventory.Discounts discount"
+ " where product.ID = :productID"
+ " and category.Style = :style"
+ " and inventory.InStock = 1");
With this query I get an list of object arrays that each have a Category entity, Inventory entity, and a DiscountAmount decimal. My goal is to use a QueryOver query to do this same query with no magic strings, but I can't get it to work. Here's what I've tried so far:
Product productAlias = null;
Category categoryAlias = null;
...
var query = session.QueryOver<Product>(() => productAlias)
.Where(() => productAlias.ID == productID)
.JoinAlias(() => productAlias.Categories, () => categoryAlias)
...
.Select(Projections.Property(() => categoryAlias.ID),
Projections.Property(() => discountAlias.Inventory),
Projections.Property(() => discount.DiscountAmount));
This query only pulls back the ID for Category, and while it does pull the full Inventory entity back it uses a full additional database query to grab it.
...
.Select(Projections.Property(() => categoryAlias),
Projections.Property(() => inventoryAlias),
Projections.Property(() => discountAlias.DiscountAmount));
This query throws a runtime exception of "Could not resolve property: categoryAlias of : Product".
...
.Select(Projections.Property(() => categoryAlias.ID).WithAlias(() => ReturnClass.Category),
Projections.Property(() => inventoryAlias.ID).WithAlias(() => ReturnClass.Inventory),
Projections.Property(() => discountAlias.DiscountAmount).WithAlias(() => ReturnClass.DiscountAmount))
.TransformUsing(Transformers.AliasToBean<ReturnClass>());
This query throws a runtime exception of "Object of type Int32 cannot be converted to type Category".
...
.Select(Projections.Property(() => categoryAlias.ID)
.TransformUsing(Transformers.AliasToBean<Category>())
This query returns a default Category entity.
So is there any way to mimic the HQL query using the QueryOver API, or is my only option to choose between HQL or making multiple queries?
Edit: To be more clear, I really want to avoid magic strings as much as possible, so I'd really prefer strongly typed QueryOver queries. Currently I'm using a QueryOver query that returns the IDs for the Category and Inventory entities and then querying for them separately, but since I have to hit those tables in the first query anyway I'd rather return them all at once.
Edit 2: The exact SQL I'm trying to achieve is
Select Category.ID, Category.CategoryName, Category.Style, (other Category columns),
Inventory.ID, Inventory.InStock, (other Inventory columns),
Discount.DiscountAmount
From Products as Product
Inner join Categories as Category ...
Where Product.ID = #productID
And Category.Style = #style
And ...

Answer 1

I think you should use Fetch instead of JoinAlias:
.Fetch(product => product.Categories).Eager
and don't use select: .List<Product>() // then by LINQ you can get Categories and Inventories from Product

Answer 2

So here is an example of "eager" loading all of the subcategories for my categories.
No N+1 when iterating over the categories collection. Future is the key here.
Category catalias = null;
var subCategories =_session.QueryOver<Category>().JoinQueryOver(x => x.SubCategories, () => catalias, JoinType.LeftOuterJoin).
Future<Category>();
var categories = _session.QueryOver<Category>().Where(x => x.ParentCategoryId == null).Future<Category>();