Relationships fixup in EntityFramework vs NHibernate

Question

I have been experimenting with Entity Framework 4.4, NHibernate 3.3.1.4000 and SQL Server and I have noticed a difference when it comes to fixing up the relationships when you commit your changes, and I was wondering what is the best practice or if I'm doing something wrong.

Here's what I tested. I have a classic Parent linked to n Children. I have 2 parents in the database with 20 children each. I load both parents, and I take the first child of the first parent and assign that child the second parent. I then commit the changes.

In EF, after I have saved, I can see that the count for the Children collection of both parents has been altered, so it fixed up the relationships.

However, when I do the same thing in NHibernate, the counts remain the same.

Here's my code setup to reproduce the issue.

POCOs:

public class Parent
{
    public virtual int ParentId { get; set; }

    public virtual string Name { get; set; }

    public virtual IList<Child> Children { get; set; }

    public Parent()
    {
        Children = new List<Child>();
    }
}

public class Child
{
    public virtual int ChildId { get; set; }

    public virtual int ParentId { get; set; }

    public virtual string Name { get; set; }

    public virtual Parent Parent { get; set; }
}

NHibernate Parent mapping:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="ConsoleApplication1"
                   namespace="ConsoleApplication1">
  <class name="Parent" table="Parents">
    <id name="ParentId" column="ParentId" />
    <property name="Name" column="Name" />
    <bag name="Children" cascade="all">
      <key column="ParentId"/>
      <one-to-many class="Child"/>
    </bag>
  </class>
</hibernate-mapping>    

NHibernate Child mapping:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="ConsoleApplication1"
                   namespace="ConsoleApplication1">
  <class name="Child" table="Children">
    <id name="ChildId" column="ChildId" />
    <property name="Name" column="Name" />
    <many-to-one name="Parent" class="Parent" column="ParentId" not-null="true" />
  </class>
</hibernate-mapping>    

EF DbContext:

public class Entities : DbContext
{
    public DbSet<Parent> Parents { get; set; }

    public DbSet<Child> Children { get; set; }
}

App.config:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <section name="hibernate-configuration" type="NHibernate.Cfg.ConfigurationSectionHandler,NHibernate" />
  </configSections>
  <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2">
    <session-factory>
      <property name="connection.provider">
        NHibernate.Connection.DriverConnectionProvider
      </property>
      <property name="dialect">
        NHibernate.Dialect.MsSql2005Dialect
      </property>
      <property name="connection.driver_class">
        NHibernate.Driver.SqlClientDriver
      </property>
      <property name="connection.connection_string">
        Data Source=localhost;Initial Catalog=MaintainRelationshipsNH;Integrated Security=True;
      </property>
    </session-factory>
  </hibernate-configuration>
  <connectionStrings>
    <add
      name="Entities"
      providerName="System.Data.SqlClient"
      connectionString="Server=localhost;Database=MaintainRelationshipsNH;Trusted_Connection=true;"/>
  </connectionStrings>
</configuration>

Script to create the tables and data:

create table Parents (
   ParentId INT not null,
   Name NVARCHAR(255) null,
   primary key (ParentId)
)

create table Children (
   ChildId INT not null,
   Name NVARCHAR(255) null,
   ParentId INT not null,
   primary key (ChildId)
)

alter table Children
    add constraint FK_Children_Parents
    foreign key (ParentId)
    references Parents

declare @idChild int        
insert into Parents (ParentId, Name) values (0, 'John');
set @idChild = 0
while @idChild < 20
begin
   insert into Children (ChildId, Name, ParentId) values (@idChild, 'Child ' + convert(nvarchar(2), @idChild), 0);
   set @idChild = @idChild + 1
end
insert into Parents (ParentId, Name) values (1, 'Julie');
while @idChild < 40
begin
   insert into Children (ChildId, Name, ParentId) values (@idChild, 'Child ' + convert(nvarchar(2), @idChild), 1);
   set @idChild = @idChild + 1
end

NHibernate test code:

System.Diagnostics.Debug.WriteLine("Test NHibernate:");

Configuration configuration = new Configuration();
configuration.Configure();
configuration.AddAssembly(typeof(Parent).Assembly);

ISessionFactory sessionFactory = configuration.BuildSessionFactory();

Parent parent0, parent1;
using (ISession session = sessionFactory.OpenSession())
{
    using (ITransaction transaction = session.BeginTransaction())
    {
        parent0 = session.Load<Parent>(0);
        parent1 = session.Load<Parent>(1);
        System.Diagnostics.Debug.WriteLine("Before modifications and commit");
        System.Diagnostics.Debug.WriteLine("Parent0 number of children: " + parent0.Children.Count);
        System.Diagnostics.Debug.WriteLine("Parent1 number of children: " + parent1.Children.Count);

        parent0.Children[0].Parent = parent1;

        transaction.Commit();
    }
}
System.Diagnostics.Debug.WriteLine("After modifications and commit");
System.Diagnostics.Debug.WriteLine("Parent0 number of children: " + parent0.Children.Count);
System.Diagnostics.Debug.WriteLine("Parent1 number of children: " + parent1.Children.Count);

Entity framework test code:

System.Diagnostics.Debug.WriteLine("Test Entity Framework:");

Parent parent0, parent1;
using (Entities entities = new Entities())
{
    parent0 = entities.Parents.Find(0);
    parent1 = entities.Parents.Find(1);
    System.Diagnostics.Debug.WriteLine("Before modifications and commit");
    System.Diagnostics.Debug.WriteLine("Parent0 number of children: " + parent0.Children.Count);
    System.Diagnostics.Debug.WriteLine("Parent1 number of children: " + parent1.Children.Count);
    parent0.Children[0].Parent = parent1;

    entities.SaveChanges();
}
System.Diagnostics.Debug.WriteLine("After modifications and commit");
System.Diagnostics.Debug.WriteLine("Parent0 number of children: " + parent0.Children.Count);
System.Diagnostics.Debug.WriteLine("Parent1 number of children: " + parent1.Children.Count);

So basically with this test I can see the counts changing with EF but not with NHibernate. Am I doing something wrong with NH or do I have to manually manage every part of the relationships affected by my changes ? Thanks!

Answer 1

In NHibernate you would need to manually move them from one parent collection to another. I usually use Add or Remove methods in the parent classes to do this. Here is an example of these add or remove methods:

    public virtual void AddLine(OrderLine orderLine)
    {
        orderLine.Order = this;
        this.orderLines.Add(orderLine);
    }

    public virtual void RemoveLine(OrderLine orderLine)
    {
        this.orderLines.Remove(orderLine);
    }

To reparent a child I would then do something like this:

    originalParent.RemoveLine(child);

    newParent.AddLine(child);