Exception deleting child records in NHibernate

Question

I have a simple parent/child tables. Contract is the parent table. Each contract can have multiple units.

Here is my C# class definitions (simplified):

    public class Contract : EntityWithIntID
{
    public virtual string ContractNum { get; set; }
    public virtual IList<Unit> Units { get; protected set; }
    public virtual int NumberOfUnits
    {
        get { return Units.Count; }
    }
 }

    public class Unit : EntityWithIntID
{
    <various Unit physical data fields>
}

I'm using FluentNHibernate with AutoMapping. Here is my AutoMapping class:

   public static AutoPersistenceModel GetMappings()
    {
        AutoPersistenceModel returnModel = AutoMap.AssemblyOf<Contract>()
            .IgnoreBase(typeof(EntityWithIntID))
            .Where(type => type.BaseType == typeof(EntityWithIntID) )
            .Conventions.Add(typeof(PluralTableNamesConvention))
            .Conventions.Add(typeof(CascadeAllConvention))
            .Override<Contract>(map =>map.HasMany(cont =>cont.Units).Inverse())
            .Override<Contract>(map=>map.Map(cont => cont.ContractNum).Not.Nullable().Unique())
        ;
        return returnModel;
    }
}

Here are the HBM.XML files that Fluent generates:

(for the Units table):

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="property" auto-import="true" default-cascade="none" default-lazy="true">
<class xmlns="urn:nhibernate-mapping-2.2" name="InterfaceDB.Unit, InterfaceDB, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="Units">
   <id name="Id" type="System.Int32, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Id" />
      <generator class="hilo">
        <param name="max_lo">10</param>
      </generator>
   </id>
<!-- physical data property elements removed for brevity -->
  </class>
</hibernate-mapping>

(and for the Contracts table):

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-access="property" auto-import="true" default-cascade="none" default-lazy="true">
  <class xmlns="urn:nhibernate-mapping-2.2" name="InterfaceDB.Contract, InterfaceDB, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" table="Contracts">
    <id name="Id" type="System.Int32, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="Id" />
      <generator class="hilo">
        <param name="max_lo">10</param>
      </generator>
    </id>
    <property name="ContractNum" type="System.String, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089">
      <column name="ContractNum" not-null="true" unique="true" />
    </property>
    <bag cascade="all" inverse="true" name="Units">
      <key>
        <column name="Contract_id" />
      </key>
      <one-to-many class="InterfaceDB.Unit, InterfaceDB, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
    </bag>
  </class>
</hibernate-mapping>

For my unit testing purposes, I have a "Clear()" method in each of the repositories so I can start with a known empty data base. It simply does a

Session.Delete("from Unit");

followed by a Debug.Assert(Repository.GetCount() == 0);

My problem is that when I do a UnitsRepository.Clear(), I get an NHibernate.ObjectDeleteException: deleted object would be re-saved by cascade (remove deleted object from associations)

I have googled that error, and found a number of articles about it, but nothing suggested seems to work. I have added the ".Inverse()" on the parent mapping. I have tried changing the cascading from "All" to "SaveUpdate" (it was never set to "AllDeleteOrphans", which some posts cited as the problem.) I'm clearing all the repositories, and I tried wrapping that whole thing in a transaction. I tried adding a flush after the Session.Delete. I tried clearing the parent repository first, then the child. Nothing gets rid of the error.

Any help would be appreciated.

Answer 1

You don't show it in your mappings but presumably Unit has a reference to its parent Contract. In order to delete a Unit you have to set the reference to the Contract to null and, depending on your mapping, remove it from the Contract's Units collection.