Question

In my quest for a version-wide database filter for an application, I have written the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using FluentNHibernate.Automapping;
using FluentNHibernate.Automapping.Alterations;
using FluentNHibernate.Mapping;
using MvcExtensions.Model;
using NHibernate;

namespace MvcExtensions.Services.Impl.FluentNHibernate
{
    public interface IVersionAware
    {
        string Version { get; set; }
    }

    public class VersionFilter : FilterDefinition
    {
        const string FILTERNAME = "MyVersionFilter";
        const string COLUMNNAME = "Version";

        public VersionFilter()
        {
            this.WithName(FILTERNAME)
                .WithCondition("Version = :"+COLUMNNAME)
                .AddParameter(COLUMNNAME, NHibernateUtil.String );
        }

        public static void EnableVersionFilter(ISession session,string version)
        {
            session.EnableFilter(FILTERNAME).SetParameter(COLUMNNAME, version);
        }

        public static void DisableVersionFilter(ISession session)
        {
            session.DisableFilter(FILTERNAME);
        }
    }

    public class VersionAwareOverride : IAutoMappingOverride<IVersionAware>
    {
        #region IAutoMappingOverride<IVersionAware> Members

        public void Override(AutoMapping<IVersionAware> mapping)
        {
            mapping.ApplyFilter<VersionFilter>();
        }
        #endregion
    }

}

But, since overrides do not work on interfaces, I am looking for a way to implement this.
Currently I’m using this (rather cumbersome) way for each class that implements the interface :

public class SomeVersionedEntity : IModelId, IVersionAware
{
    public virtual int Id { get; set; }
    public virtual string Version { get; set; }
}

public class SomeVersionedEntityOverride : IAutoMappingOverride<SomeVersionedEntity>
{
    #region IAutoMappingOverride<SomeVersionedEntity> Members

    public void Override(AutoMapping<SomeVersionedEntity> mapping)
    {
        mapping.ApplyFilter<VersionFilter>();
    }

    #endregion
}

I have been looking at IClassmap interfaces etc, but they do not seem to provide a way to access the ApplyFilter method, so I have not got a clue here…

Since I am probably not the first one who has this problem, I am quite sure that it should be possible; I am just not quite sure how this works..

EDIT :
I have gotten a bit closer to a generic solution:

This is the way I tried to solve it :

Using a generic class to implement alterations to classes implementing an interface :

public abstract class AutomappingInterfaceAlteration<I> : IAutoMappingAlteration
{
    public void Alter(AutoPersistenceModel model)
    {
        model.OverrideAll(map =>
        {
            var recordType = map.GetType().GetGenericArguments().Single();
            if (typeof(I).IsAssignableFrom(recordType))
            {
                this.GetType().GetMethod("overrideStuff").MakeGenericMethod(recordType).Invoke(this, new object[] { model });
            }
        });
    }

    public void overrideStuff<T>(AutoPersistenceModel pm) where T : I
    {
        pm.Override<T>( a => Override(a));
    }

    public abstract void Override<T>(AutoMapping<T> am) where T:I;
}

And a specific implementation :

public class VersionAwareAlteration : AutomappingInterfaceAlteration<IVersionAware>
{
    public override void Override<T>(AutoMapping<T> am)
    {
        am.Map(x => x.Version).Column("VersionTest");
        am.ApplyFilter<VersionFilter>();
    }
}

Unfortunately I get the following error now :

[InvalidOperationException: Collection was modified; enumeration operation may not execute.]
   System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) +51
   System.Collections.Generic.Enumerator.MoveNextRare() +7661017
   System.Collections.Generic.Enumerator.MoveNext() +61
   System.Linq.WhereListIterator`1.MoveNext() +156
   FluentNHibernate.Utils.CollectionExtensions.Each(IEnumerable`1 enumerable, Action`1 each) +239
   FluentNHibernate.Automapping.AutoMapper.ApplyOverrides(Type classType, IList`1 mappedProperties, ClassMappingBase mapping) +345
   FluentNHibernate.Automapping.AutoMapper.MergeMap(Type classType, ClassMappingBase mapping, IList`1 mappedProperties) +43
   FluentNHibernate.Automapping.AutoMapper.Map(Type classType, List`1 types) +566
   FluentNHibernate.Automapping.AutoPersistenceModel.AddMapping(Type type) +85
   FluentNHibernate.Automapping.AutoPersistenceModel.CompileMappings() +746

EDIT 2 : I managed to get a bit further; I now invoke “Override” using reflection for each class that implements the interface :

public abstract class PersistenceOverride<I> 
{

    public void DoOverrides(AutoPersistenceModel model,IEnumerable<Type> Mytypes)
    {
        foreach(var t in Mytypes.Where(x=>typeof(I).IsAssignableFrom(x)))
            ManualOverride(t,model);
    }

    private void ManualOverride(Type recordType,AutoPersistenceModel model)
    {
        var t_amt = typeof(AutoMapping<>).MakeGenericType(recordType);
        var t_act = typeof(Action<>).MakeGenericType(t_amt);
        var m = typeof(PersistenceOverride<I>)
                .GetMethod("MyOverride")
                .MakeGenericMethod(recordType)
                .Invoke(this, null);
        model.GetType().GetMethod("Override").MakeGenericMethod(recordType).Invoke(model, new object[] { m });
    }

    public abstract Action<AutoMapping<T>> MyOverride<T>() where T:I;
}

public class VersionAwareOverride : PersistenceOverride<IVersionAware>
{
    public override Action<AutoMapping<T>> MyOverride<T>()
    {
        return am =>
        {
            am.Map(x => x.Version).Column(VersionFilter.COLUMNNAME);
            am.ApplyFilter<VersionFilter>();
        };
    }
}

However, for one reason or another my generated hbm files do not contain any “filter” fields….
Maybe somebody could help me a bit further now ??

Answer 1

Apparently, there was a bug in the current version of fluent-nhibernate. The filters did not get copied when you use Automapper.Override<T> .

I forked the source, fixed the bug, tested, and I have now sent a pull request to the fluentnhib guys over at github.

For now, you can download the fixed sourcecode at http://github.com/ToJans/fluent-nhibernate/commit/29058de9b2bc3af85bc433aa6f71549f7b5d8e04

There is now also a complete blog post on how to do this :
http://www.corebvba.be/blog/post/How-to-override-interface-mappings-and-creata-a-generic-entity-version-filter-in-fluent-nhibernate.aspx