NHibernate UniqueResult alternative?

Question

We're using NHibernate in a project that gets data out of the database and writes reports to a separate system. In my scenario, a patient will usually, but not always, have a next appointment scheduled when the report gets written. The query below gets the next appointment data, to include in the report.

private NextFollowup GetNextFollowup(int EncounterID)
    {
        try
        {
            NextFollowup myNextF = new NextFollowup();

            IQuery myNextQ = this.Session.GetNamedQuery("GetNextFollowup").SetInt32("EncounterID", EncounterID);

            myNextF = myNextQ.UniqueResult<NextFollowup>();

            return myNextF;

        }
        catch (Exception e)
        {
            throw e;
        }

    }

Here's the question:

Usually this works fine, as there is a single result when an appointment is scheduled. However, in the cases where there is no next followup, I get the error that there is no unique result. I don't really want to throw an exception in this case, I want to return the empty object. If I were to get a list instead of a UniqueResult, I'd get an empty list in the situations where there is no next followup. Is there a better way to handle the situation of "when there is a value, there will be only one" than using a list in the HQL query?

Trevor Pilley Trevor Pilley · Accepted Answer · 2012-03-14T13:15:29

This may work:

private NextFollowup GetNextFollowup(int encounterID)
{
    IQuery query = this.Session.GetNamedQuery("GetNextFollowup").SetInt32("EncounterID", encounterID);

    // nextFollowup will be either the next instance, or null if none exist in the db.
    var nextFollowup = query.Enumerable<NextFollowup>().SingleOrDefault();

    return nextFollowup;
}

Note: updated naming to follow Microsoft best practices

The try catch is not serving any purpose here except to loose the stack trace if there is an exception so I've removed it.

If you want to return a new NextFollowup if none exist, you can update the query line to:

var nextFollowup = query.Enumerable<NextFollowup>().SingleOrDefault() ?? new NextFollowup();

NHibernate UniqueResult alternative?

1 Answers