Re: mapping 1 to 0 or 1 relationships to null objects

[Peter Mills]

Hi Ka-Wai,

I find the easiest way to do that kind of mapping is in the domain 
object, itself.

public class Customer {
   SubObject _subObject = SubObject.Empty;

   public SubObject MySubObject {
      get { return _subObject; }
      set {
         if (null == value)
            _subObject = SubObject.Empty;
         else
            _subObject = value;
      }
   }
}

It also protects you from accidentally setting the property null, 
somewhere else in your code.

Cheers,

Peter

Ka-Wai Chan wrote:
Hello
 
I have a 1 to 0 or 1 relationship mapped as follow
 
      <result property="SubObject" column="sub_object_id" 
select="SelectBySubtObjectId"/>
 
This works fine, but what we are trying to implement is the NullObject 
pattern in which a null value is mapped to a specific null object 
instead of null.
 
ie.  In Parent object, I have a reference called SubObject.  Ibatis 
right now will set that reference to null when a database value of 
null is encountered, what I want is for Ibatis to set that value to 
SubObject.NULL instead.
 
Is this possible? What is the proper way of doing this? 
 
I have tried implementing ITypeHandlerCallback by doing the following 
but doesn't work.  When is the NullValue getter called?
 
        public void SetParameter(IParameterSetter setter, object 
parameter)
        {
        }
 
        public object GetResult(IResultGetter getter)
        {
            return getter.Value;
        }
 
        public object ValueOf(string s)
        {
            return s;
        }
 
        public object NullValue
        {
            get { return SubObject.NULL; }
        }
 
Thanks,
Ka-Wai