Question

I have a parent entity with a list of child entities. When using NHibernate to retrieve a given parent with children from SQL, it works fine if there are no children OR if there are children with dates that match the where condition.

If there are children that do not match the where clause, the parent is null. I want to have the parent initialized with an empty list of children.

Any thoughts on how I can modify the code below to make this happen?

Entities:

public class Parent
{
    public int ParentId;
    public IList<Child> Children { get; set; }

    public Parent()
    {
        Children = new List<Child>();
    }
}

public class Child
{
    public int ChildId;
    public DateTime ChildDate;
    public Parent Parent { get; set; }
}

Repository:

IList<Parent> foundParents = new List<Parent>();

var criteria1 = DetachedCriteria.For<Parent>()
    .Add(Restrictions.Eq("ParentId", parentId))
    .CreateCriteria("Children", JoinType.LeftOuterJoin)
        .Add(Restrictions.Or(
            Restrictions.IsNull("ChildDate"), // no children at all
            Restrictions.And(
                Restrictions.Ge("ChildDate", startDate),
                Restrictions.Le("ChildDate", endDate)
            )
        ));

foundParents = Session
    .CreateMultiCriteria()
    .Add<Parent>(criteria1)
    .SetResultTransformer(new DistinctRootEntityResultTransformer())
    .List()[0] as List<Parent>;

If I were writing SQL for this, I would put the date comparison in with the left join and not in the where clause. I can’t figure out how to do this with NHibernate.

Answer 1

This took alot of research – the key to finding an answer is the term filter. I came across the term by digging through the NHibernate source code starting with AddJoin in ANSIJoinFragment.cs – the code supported additional conditions on the join so I figured it was possible.

Anyway, here’s the revised code that utilizes filter (entity class remains the same).

Repository:

IList<Parent> foundParents = new List<Parent>();

var criteria1 = DetachedCriteria.For<Parent>()
    .Add(Restrictions.Eq("ParentId", parentId))
    .CreateCriteria("Children", JoinType.LeftOuterJoin);

Session.EnableFilter("dateFilter")
    .SetParameter("startDate", startDate)
    .SetParameter("endDate", endDate);

foundParents = Session
    .CreateMultiCriteria()
    .Add<Parent>(criteria1)
    .SetResultTransformer(new DistinctRootEntityResultTransformer())
    .List()[0] as List<Parent>;

I also had to modify my mapping for Parent by adding filter and filter-def elements.

<class name="Parent" table="Parents">

  ...
  <bag name="Children" table="Children">
    ...
    <filter name="dateFilter" 
      condition="ChildDate BETWEEN :startDate and :endDate" />
  </bag>
</class>

<filter-def name="dateFilter">
  <filter-param name="startDate" type="System.DateTime" />
  <filter-param name="endDate" type="System.DateTime" />
</filter-def>

Also, one word of warning for anyone that runs into this problem and doesn’t use filters. If you decide to return the Parent entity without the populated children when the original query with the where clause yields no records, any code that hits the set of children will cause NHibernate to load the entire table.