NHibernate QueryOver - Retrieve all, and mark the ones already "selected"

Question

Dear NHibernate experts,

The following query gives me all my categories:

var result = Session.QueryOver(() => cat).List();

.. and by running this query, I get the ones selected (category_x_product table):

int productId = 11;
Category cat = null;
CategoryProduct cp = null;

var subQuery = QueryOver.Of(() => cp)
        .Where(() => cp.ProductId == productId)
        .Select(Projections.Distinct(Projections.Property(() => cp.CategoryId)));

result = Session.QueryOver(() => cat)
                .WithSubquery
                .WhereProperty(() => cat.Id).In(subQuery)
                .List();

Any way to combine those two queries, so that I get all categories with a boolean value indicating which one was in fact "selected" in the CategoryProduct-query.

Map it to an entity like this, maybe?

CategorySelected
----------------
Category Category { get; set; }
bool IsSelected { get; set;

I've tried to find an answer to this, using QueryOver, but didnt succeed. Is this even possible in a "more or less" simple query? Any help is much appreciated. Thanks!

mikal

Answer 1

One way to achieve that, is to create Conditional SELECT statement. In Case of SQL Server we would like to generate something like this

SELECT CASE CategoryId IN (.... subselect ) THEN 1 ELSE 0 END ...

But thanks to NHibernate, and abstract Querying API, we can create the query to be working in all supported DB dialects.

Let's try to create a draft of new solution. We will adjust the SubQuery first

var subQuery = QueryOver.Of(() => cp)
    .Select(Projections.Distinct(Projections.Property(() => cp.CategoryId)));

Now we will create the conditional statement

var isSelected = Projections.Conditional(
    Subqueries.PropertyIn("Id", subQuery) // Category ID to be in the inner select
    , Projections.Constant(1)
    , Projections.Constant(0)
);

And we will inject that condition into the QueryOver and use Transformers to have correctly populated properties of the Category (including the virtual IsSelected)

Category category = null
result = Session.QueryOver(() => cat)
    // SELECT clause is now built
    .SelectList(list => list
        .Select(isSelected).WithAlias(() => category.IsSelected)
        .Select(ca => ca.Id).WithAlias(() => category.Id)
        ... // all properites we would like to be populated
    )
    // Transform results into Category again
    .TransformUsing(Transformers.AliasToBean<Category>())
    .List<Category>();

And now, Our new IsSelected property, which is not mapped, but only used for this SELECT (projections), is populated with the correct information.

NOTE: this approach is working, but the statements should be taken as a draft. Some adjustment could be needed in your case...