nhibernate many-to-one parent is always null on insert - wcf

Question

I am using NHibernate 3.1.0.4000 and AutoMapper 2.0.0.0 in a WCF. I have a parent-child relationship I want to maintain from the "many" end. I have no problems maintaining the objects if I do it from the "one" end but in this case that does not make sense. My issue is no matter how I change my mappings, POCOs, etc. the parent object when I attempt to add a child is null in the child causing the insert to fail. What am I missing to get the parent property in the child to populate?
I have a parent-child relationship defined in the following tables:
Create Table Attribute (AttributeUID uniqueidentifier, LongName varchar(20))
Create Table AnswerOption (AnswerOptionID int, AttributeUID uniqueidentifier)
I want the Attribute (parent) to be the owner so I declare the relationship in that mapping file and not in the AnswerOption (child). Though, I have tried with having the relationship bidirectional as well and that has not changed any behaviors in my tests. My mappings apear as follows. Attribute:
<class name="RCAttribute" table="rcs.tblAttribute">
<cache usage="read-write"/>
<id name="ID">
<column name="AttributeUID" />
<generator class="guid" />
</id>
<property name="LongName" type="string" not-null="true" length="200" column="LongName" />
<bag name="AnswerOptions" lazy="true" inverse="true" cascade="all">
<key column="AttributeUID"/>
<one-to-many class="AnswerOption" />
</bag>
</class>
AnswerOption:
<class name="AnswerOption" table="rcs.tblAnswerOption" lazy="true">
<cache usage="read-write"/>
<id name="ID">
<column name="AnswerOptionID" />
<generator class="native" />
</id>
</class>
Attribute Class:
[Serializable]
public class RCAttribute
{
public virtual Guid ID { get; set; }
public virtual string LongName { get; set; }
public virtual ICollection<AnswerOption> AnswerOptions { get; set; }
public RCAttribute() { ID = new Guid("00000000-0000-0000-0000-000000000000"); }
}
AnswerOption Class:
[Serializable]
public class AnswerOption
{
public virtual int ID { get; set; }
public AnswerOption() { ID = 0; }
}
My test procedure looks like this;
public void CreateAnswerOption()
{
AnswerOption newOpt = new AnswerOption();
Attribute.AnswerOptions.Add(newOpt);
Attribute = rc.RCAttributeSave(Attribute);
}
When it goes to create this the Attribute property of the AnswerOption is null so it cannot insert since the parent cannot be null in the child in this case. What am I missing to get it to populate the parent property on the child and be able to insert?

Answer 1

Mapping the collection as inverse without mapping the many-to-one on the other side makes no sense.
You have three options:
Use a bidirectional mapping with inverse="true" on the collection side and set the many-to-one propery (parent reference) in your code before saving the child (yes, I read that you don't want to do it that way).
Only map the collection side (not inverse). NHibernate will then first insert the child with NULL as parent reference, but will update it with the correct parent ID in the same transaction. So you can't have a not null constraint on the parent ID column in the child table (at least it must be deferrable).
(This option only works with NHibernate 3.2.0 or newer) Same as option 2, but add not-null="true" to the key tag in the collection mapping. Then NHibernate will insert the child with the parent ID already set.