How to do an NHibernate QueryOver with “not in”?

Question

Is there a way to make this query easier with NHibernate?

For the understanding, this is the query which I want to create with NHibernate:

Select * from Task
Where task_id not in
(Select task_id from UserTask
Where solved = 1 AND [user_id] = 1)

And this one is my Code in C# with NHibernate

public IList<Task> RandomTasks(List<int> subject_ids, int userId)
{
    //Gets all Ids where the User has solved the Task
    List<int> task_ids = new List<int>(_session.Query<UserTask>()
                       .Where(x => x.User.user_id == userId)
                       .Where(x => x.solved)
                       .Select(x => x.Task.task_id));

    //Gets all Tasks except the task_ids from the result of the query befor
    var query = _session.QueryOver<Task>()
                       .Where(s => s.Subject.subject_id.IsIn(subject_ids))
                       .Where(t => !t.task_id.IsIn(task_ids))
                       .Take(10)
                       .List();

    return query;
}

The query returns the correct result but I think there is maybe an easier way to get the same result.

Answer 1

If you would prefer INNER SELECT , NHiberante does have the solution for you. It is called DetachedCriteria . (try to check here for similar example)

So firstly we will create the inner select:

var sub = DetachedCriteria
  .For<UserTask>()
  // WHERE 
  .Add(Restrictions.In("UserId", new int[] { 1, 2 })) // example of filtering 'user_id'
  .Add(Restrictions.Eq("solved", true))               // 'solved'
  .SetProjection(Projections.Property("TaskId")); // TaskId the SELECT clause

(I am not sure about your model and naming eg task_id vs TaskId ... but the intention should be clear)

With DetachedCritera we can do a lot, we can join other referenced objects/tables, filter them... as with standard QueryOver. The only difference is, that we should return Projection (SELECT TaskId) to be used as a filter in another query:

var criteria = session.CreateCriteria<Task>();
criteria
  . Where(...                             // your filters applied on Task
  .Add(Subqueries.PropertyIn("ID", sub)); // Task.ID in (select...

var result = criteria.List();

NOTE. This solution will not only work ;), but mostly, it is ready for paging and sorting . So even in cases, that the inner select will return more 'similar' results (same ids), the upper select will return each task only once...

For more information: 15.8. Detached queries and subqueries

Answer 2

You can use a left join :

Select t.* from Task t
left join UserTask ut on t.Id = ut.TaskId 
    and ut.UserId = 1 and ut.solved = 1
where ut.id is null