Preventing multiple instance after inner join - nhibernate

Question

I have a small problem with multiple instances of the same object after a join to an other table. For testing I create one Store with two Products (ManyToMany-Relation). The following snippet hopefully describes my problem.
var preResult = _session.QueryOver<Store>().List(); // One store
Product productAlias = null;
var result = _session.QueryOver<Store>()
.JoinAlias(s => s.Products, () => productAlias)
.List(); // Two instances of the same store
I even think this behavior is correct but how can I prevent the multiple instances? Is it possible within the query?
Just for information why I need to make this unnecessary join: I want to extend the query according to different critirias, similar to this:
Product productAlias = null;
var query = _session.QueryOver<Store>().JoinAlias(s => s.Products, () => productAlias);
if (!string.IsNullOrWhiteSpace(criteria.ProductName))
{
query.Where(Restrictions.On(() => productAlias.Name).IsInsensitiveLike(criteria.ProductName));
}
if (criteria.ProductType != null)
{
query.Where(s => productAlias.Type == criteria.ProductType);
}
var result = query.List();
Here I ran into different problems, depending on the criterias.

Answer 1

Try using Transformers.DistinctRootEntity in your scenario to eliminate the cartesian product.
Product productAlias = null;
var query = _session.QueryOver<Store>()
.JoinAlias(s => s.Products, () => productAlias)
query = query.TransformUsing(Transformers.DistinctRootEntity);
var result = query.List();

Answer 2

Let's split solution into two queries.
Top one QueryOver<Store>() will be correctly returning just a distinct list. And what's more, by design it will support paging (Take(), Skip()).
The inner one, will be returning just a list of Store IDs, which fully meet whatever criteria...
The result SQL will look like this
SELECT ... // top one
FROM Store
WHERE StoreID IN ( SELECT StoreID ...) // inner one
Inner
Let's start with the inner select, the NHibernate detached QueryOver:
Store storeAlias = null;
Product productAlias = null;
// detached query, resulting in a set of searched StoreID
var subQuery = QueryOver.Of<Store>(() => storeAlias)
.JoinAlias((s) => s.Products, () => productAlias)
.Select((s) => s.ID); // ID projection
if (!string.IsNullOrWhiteSpace(criteria.ProductName))
{
subQuery.Where(Restrictions.On(() => productAlias.Code)
.IsInsensitiveLike(criteria.ProductName));
}
Top
Once we have filtered the Store we can use this subquery in top one
var query = session.QueryOver<Store>()
// IN clause
.Where(Subqueries.PropertyIn("ID", subQuery.DetachedCriteria))
.Skip(100)
.Take(50) // paging over already distinct resultset
;
var result = query.List<Store>();
And now we can apply whatever filter to inner query, and get list of Store IDs which do meet filter criteria... while working with top query, which is distinct...