nHibernate, table-per-class and querying via base class

Question

I have some class hierarchy, mapped by the code into several tables. One for base abstract class, and one table per concrete class. Classes hierarchy is relatively simple, like this:

public abstract class BaseClass {
  // some common fields here
}

public class Subclass1 : BaseClass {
}

public class Subclass2 : BaseClass {
}

public class Subclass3 : BaseClass {
}

The problem appears when I try to Query over BaseClass to retrieve a collection of all subclasses from the database.

var allInstances = dbSession.QueryOver<BaseClass>().List();

Error message is very simple: "Cannot instantiate abstract class or interface".

So my question is: is there a way to have one single query for my scenario, or I have to run one query per subclass?

Felipe Oriani Felipe Oriani · Accepted Answer · 2015-03-04T12:40:02

I am not sure about the QueryOver of Nhibernate, but you could do a polymorphic query using the Criteria.

var result = dbSession.CreateCriteria<BaseClass>()
                      .List<BaseClass>();

You also could try using the Linq

using NHibernate.Linq;

/// ...

var result = dbSession.Query<BaseClass>()
                      .List();

nHibernate, table-per-class and querying via base class

2 Answers