Bidirectional Child when there are many Parent classes

Scottpkuehn · **Joined:** Fri Jun 06, 2008 6:25 pm **Posts:** 8

I have the situation were we have many Parent classes that have a one-to-many relationship to a common Child class. I would like to have the child be bidirectional; i.e. the Child class having a reference back to the parent.

I have it working for a single Parent/Child relationship, but I am unsure how to map it when there are mulitple Parent class types from the Childs perspective.

In general, how do I do the mapping for the Child?
Are there any changes I need to make for the standard Parent mapping?

ParentClassA => (1-n) => Child
ParentClassB => (1-n) => Child

Code:

public class ParentClassA : ParentBase
{
    private IList<Child> m_children;
...
    public IList<Child> Children
    {
        get { return m_children; }
        set { m_children = value }
    }
}

public class ParentClassB : ParentBase
{
    private IList<Child> m_children;
...
    public IList<Child> Children
    {
        get { return m_children; }
        set { m_children = value }
    }
}

public class Child
{
    private ParentBase m_parent;
...
    public ParentBase Parent
    {
        get { return m_parent; }
        set { m_parent= value }
    }
}

Please help.

wolli · **Posted:** Thu Jul 03, 2008 3:11 am

ParentBase is the base class of ParentA and ParentB ? I would say, the children have to be defined there:

Code:

public class ParentBase
{
    private IList<Child> m_children;
...
    public IList<Child> Children
    {
        get { return m_children; }
        set { m_children = value }
    }
} 

Scottpkuehn · **Joined:** Fri Jun 06, 2008 6:25 pm **Posts:** 8

I agree, but what does that mean for the mapping files?
Can ParentBase be an abstract class?

wolli · **Posted:** Thu Jul 03, 2008 9:34 am

Yes, it can. have a look at the different strategies for inheritance mapping:

http://www.hibernate.org/hib_docs/nhibernate/1.2/reference/en/html/inheritance.html

Scottpkuehn · **Joined:** Fri Jun 06, 2008 6:25 pm **Posts:** 8

You seem to have pointed me to how to persist & hydrate a Parent. In our implementation nearly all our classes inherit from the same abstract base class (all 50 of them). The question was not how to polymorphically create ParentA and ParentB from the same table using discriminators, etc. ParentA and ParentB are from different tables with very different content and purposes... but they all have one-to-many reference to a common child table.

Let's say ParentA is a Customer, ParentB is an InvoiceLineItem, and the Child is a generic DiscriptiveFeatures table.

Code:

public abstract class ParentBase
{ 
    private IList<DiscriptiveFeature> m_features; 
... 
    public IList<DiscriptiveFeature> Features
    { 
        get { return m_features; } 
        set { m_features= value } 
    } 
} 

public class DiscriptiveFeature
{ 
    private ParentBase m_parent; 

    public int FeatureType {...}
    public bool Shown {...}
    public int SecurityLevel {...}
    ...
    public ParentBase Parent 
    { 
        get { return m_parent; } 
        set { m_parent= value } 
    } 
} 

public class Customer : ParentBase 
{ 
    ... // class specific properties
} 

public class InvoiceLineItem: ParentBase 
{ 
    ... // class specific properties
} 

The original question was: "How do I bidirectionaly map the DiscriptiveFeatures/child?

wolli · **Posted:** Thu Jul 03, 2008 1:05 pm

One of these strategies is "table-per-subclass" which seems to be the one you are using ? You should find some information there on how to map your case. That's why I posted that link.

Would be something like this (only the relevant stuff):

Code:

<class name="FeaturedItem" ...>
...
   <many-to-one name="Parent" column="parentid" not-null="true" />
...
</class>

<class name="Customer">
...
  <bag name="Features" ... inverse="true">
      <key column="parentid"/>
      <one-to-many class="FeaturedItem"/>
  </bag>
...
</class>

<class name="InvoiceLineItem">
...
  <bag name="Features" ... inverse="true">
      <key column="parentid"/>
      <one-to-many class="FeaturedItem"/>
  </bag>
...
</class>

Scottpkuehn · **Joined:** Fri Jun 06, 2008 6:25 pm **Posts:** 8

I did what you suggested, but now I get the error:
"An association from the table DescriptiveFeatures refers to an unmapped class: Framework.Model.Core.ParentBase".

How can I map an abstract class?

wolli · **Posted:** Tue Jul 08, 2008 2:25 am

I'm not really sure, I think you need a "any" mapping for that. Checkout the last part of the chapter I posted before. And have a look here:

http://www.ayende.com/Blog/archive/2006/06/05/7532.aspx