Greg Does IT

Deleting a one-to-many relationship in NHibernate

I have had some trouble deleting the child in a one-to-many relationship in NHibernate. My schema described a simple parent-child relationship: a parent can have multiple children and every child has exactly one parent.

In the database schema I had two tables parentTable and childTable. ParentTable had an ID column and some other columns. The childTable had an ID primary key column, some other columns and a parentId NOT NULL column which had a foreign key constraint against the ID column in the parentTable.

The NHibernate schema describing the relationship was the following:

• Parent schema (the relevant part):

• Child schema (the relevant part):

I was trying to delete the child the following way:

ParentClass parent;
ChildClass child;
parent.children.Remove(child);
session.Delete(child);
session.Save(parent);
session.Flush();

When executing the code I got an exception saying

Cannot insert the value NULL into column 'parentId', table 'childTable'; column does not allow nulls. UPDATE fails.
The statement has been terminated.

It took me some googling to find out what I was doing wrong. The NHibernate schema was incomplete. The errors were:

• I did not specify that the parentId column in the childTable cannot be NULL. This can be done using the not-null=”true” attribute
• I did not specify that the one-to-many relation was inverse even though it was: the child knew its parent but the parent did not directly know its children in the database schema. (See this article on what this inverse attribute means in more of depth).
• It is obvious that no child can exist without a parent. NHibernate will take care of deleting the child once it is removed from the children collection of the parent if the on-delete=”cascade” is set on the key element within the children collection.

The NHibernate schema that solved my problem finally is the following:

• Parent schema (the relevant part):

• Child schema (the relevant part):

This entry was posted on Wednesday, January 28th, 2009 at 2:49 am and is filed under .NET, Tips & tricks. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

4 Responses to “Deleting a one-to-many relationship in NHibernate”

1. Hello,

   I would like to take to this opportunity to ask a question about one-to-many. I ahve two classes:

   GeneralInformation and famousPlacesInLondon

   public class GeneralInformation
   {

   private int id;

   public virtual int Id
   {
   get { return id; }
   set { id = value; }
   }

   private IList famousPlacesOutLondon;

   public virtual IList FamousPlacesOutLondon
   {
   get { return famousPlacesOutLondon; }
   set { famousPlacesOutLondon = value; }
   }
   }

   public class FamousPlacesInLondon
   {
   private int id;

   public virtual int Id
   {
   get { return id; }
   set { id = value; }
   }
   private string link;

   public virtual string Link
   {
   get { return link; }
   set { link = value; }
   }
   private string title;

   public virtual string Title
   {
   get { return title; }
   set { title = value; }
   }

   }
   }

   My mapping are as follows:

   If I fill the GeneralInformation with data and do a save using NHibernate, everything gets inserted into the appropriate table, except the ForeignKey GeneralInformationId. What is wrong?

   TIA
   Yaz

   Yazid
   April 23rd, 2009

2. i Have mapping like this, and it’s doesn’t work…:/
   I don’t know why.

   in Parent

   In child

   Artur
   September 12th, 2009

3. i Have mapping like this, and it’s doesn’t work…:/
   I don’t know why.

   app.config
   ===============

   NHibernate.Dialect.MsSql2005Dialect

   NHibernate.Connection.DriverConnectionProvider

   NHibernate.Driver.SqlClientDriver

   1

   Server=art-PC;
   Database=T1;
   User ID = sa;
   Password=password;

   NHibernate.ByteCode.Castle.ProxyFactoryFactory, NHibernate.ByteCode.Castle

   True

   ===============

   in Parent

   In child

   Artur
   September 12th, 2009

4. <!–

   s
   September 12th, 2009