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

Let’s split solution into two queries.

1. Top one QueryOver<Store>() will be correctly returning just a distinct list. And what’s more, by design it will support paging (Take(), Skip()).
2. 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…