NHibernate: CreateCriteria and Exists clause

Question

How can I write the following SQL using CreateCriteria:

SELECT * FROM FooBar fb
WHERE EXISTS (SELECT FooBarId FROM Baz b WHERE b.FooBarId = fb.Id)

Answer 1

Here is how you can do it:

var fooBars = Session.CreateCriteria<FooBar>()
        .Add(Restrictions.IsNotEmpty("Bazs")).List<FooBar>();

...assuming there is a collection property (one-to-many) "Bazs" in the FooBar object.

Alternatively you could use detached criteria like that:

DetachedCriteria dCriteria = DetachedCriteria.For<Baz>("baz")
        .SetProjection(Projections.Property("baz.FooBarId"))
        .Add(Restrictions.EqProperty("baz.FooBarId", "fooBar.Id"));

var fooBars = Session.CreateCriteria<FooBar>("fooBar")
        .Add(Subqueries.Exists(dCriteria)).List<FooBar>();

Answer 2

Having just solved a related problem and eventually arrived at a solution I thought I'd share the answer here:

Assuming you want the original questions query, with an additional condition on the sub-query:

SELECT * FROM FooBar fb
WHERE EXISTS (SELECT FooBarId FROM Baz b WHERE b.FooBarId = fb.Id
              AND Quantity = 5)

Assuming you have a reference on the Baz class to the parent, called, say FooBarRef [ in Fluent Map class you'd use the References() method ], you would create the query as follows:

DetachedCriteria dCriteria = DetachedCriteria.For<Baz>("baz")
        .SetProjection(Projections.Property("baz.FooBarId"))
        .Add(Expression.EqProperty("this.FooBarId", "FooBarRef.Id"))
        .Add(Expression.Eq("baz.Quantity", 5));

var fooBars = Session.CreateCriteria<FooBar>("fooBar")
        .Add(Subqueries.Exists(dCriteria)).List<FooBar>();

I'm not 100% convinced about hard coding of the alias "this", which is the alias NHibernate automatically assigns to the root entity (table) in the query, but it's the only way I've found to reference the key of the parent query's table from within the sub-query.