How to reference ID property in relationship with NHibernate? - nhibernate

Question

How do I map relationship, where child endpoint is exposed via Id property and not via whole Parent object?
Here is the example:
class Parent {
public Guid Id { get; set; }
public List<Child> Chlidren { get; set; }
}
class Child {
public Guid Id { get; set; }
public Guid ParentId { get; set; }
}
Here are the equivalent mappings I'm using:
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
assembly="Blabla"
namespace="Blabla"
auto-import="false">
<typedef name="ChildrenList" class="Blabla" />
<class name="Parent" table="Parent" lazy="false">
<id name="Id" column="ID" type="Guid">
<generator class="guid" />
</id>
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false">
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
</class>
<class name="Child" table="Child" lazy="false">
<id name="Id" column="ID" type="Guid">
<generator class="guid" />
</id>
<!-- How to map ParentID here? -->
</class>
</hibernate-mapping>
When I create a parent, add some children to Children collection and then save the parent, everything is fine. But if save a parent object first, then create a child, setting its ParentID property to ID of the parent, then I get
NHibernate.PropertyValueException:
not-null property references a null or transient value Child._Parent.ChildrenBackref
All attempts to map many-to-one relationship resulted in different exceptions while creating NHibernate configuration. Mostly about object type mismatch.
I'm sure NHibernate is capable to handle this scenario. There must something fairly basic that I miss.
EDIT:
I think it make sense to the example test, which fails with above exception:
var child = new Child(Create.Saved<Parent>().Id); // this sets the ParentId property
this.Repository.Save(child); // here I get the exception
My thoughts why NHibernate is raising this: Children property of Parent class mapped in a way that says that a child cannot exist without a parent (<key column="ParentID" not-null="true" />). When I try to persist a child, NHibernate tries to resolve this relationship (to find a parent this child relates to) and fails, since being given no child endpoint (which otherwise would be ParentId property) in the mapping, it check for its own Child._Parent.ChildrenBackref endpoint, whatever it is.
This looks like a desired solution: Mapping ParentId property as child endpoint of the relationship. This would force NHibernate to resolve a parent by using value of ParentId property as parent's primary key.
The thing is I don't know if it's possible.

Answer 1

The one-to-many / many-to-one relationships you have in NHibernate always needs to have a dominant side (i.e. the side that manages the "saving").
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false">
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
The above is a one-to-many relationship where the dominant side is the parent. That means, you save the parent ... and that will save the parent first, then, the children (with the ParentId being null), then a subsequent update will be issued to set the child.ParentId.
Note:
The child is inserted first with ParentId=null ... if you have a db or mapping restriction to say ParentId cannot be null, this action will fail.
<bag name="Children" table="Child"
cascade="save-update"
collection-type="ChildrenList"
lazy="false"
inverse=true>
<key column="ParentID" not-null="true" />
<one-to-many class="Child" />
</bag>
Note the inverse=true attribute. This means the child object is dominant in the relationship, meaning the child object is in charge. The parent will be inserted, then the Id will be assiged to the child.ParentId, and then the child will be inserted with the ParentId already set.
In many cases, of course, you want to go either way. The easiest way to do this is to manage the relationship on both ends (unfortunately, you have to do this yourself).
On the Parent, you have a method:
public void AddChild(Child child)
{
Children.Add(child);
child.ParentId = Id;
}
public void RemoveChild(Child child)
{
Children.Remove(child);
child.ParentId = null;
}
On the Child, you have a method:
public void SetParent(Parent parent)
{
ParentId = parent.Id;
parent.Children.Add(this);
}
Using these methods to Add/Remove/Set, both sides are consistent after the action is performed. It, then, wouldn't matter whether you set inverse=true on the bag or not.
see http://www.nhforge.org/doc/nh/en/index.html#collections-example