nhibernate "cascade="all-delete-orphan" error

Question

i have 3 tables in my database:

1. Projects (id, name)
2. Tags (id, name)
3. ProjectsTagss (id, projectId, tagid)

As you can see the ProjectsTags table is a bridge table

here is my fluent nhibernate mapping

ProjectMap.cs:

 Map(x => x.Name).Not.Nullable();
 HasMany(x => x.ProjectsTags).AsBag().Inverse()
    .Cascade.AllDeleteOrphan().Fetch.Select().BatchSize(80);

ProjectsTagsMap.cs:

 References(x => x.Project).Not.Nullable();
 References(x => x.Tag).Not.Nullable();

TagMap.cs:

  Map(x => x.Name).Not.Nullable();

As you can see, i historically didn't have the Tag table linked to anything else. I now need to generate a report to show Tag and how often that tag is used so i need to join from Tag to ProjectsTag. i tried adding this line into the tagsmap:

 HasMany(x => x.ProjectsTags).AsBag().Inverse()
    .Cascade.AllDeleteOrphan().Fetch.Select().BatchSize(80);

but when i go to update the name on a tag object and commit, i get this error:

A collection with cascade="all-delete-orphan" was no longer referenced by the owning entity instance

can anyone see anything wrong with what i added that would be causing this nhibernate exception when i simply update the Tag table. Again my goal is to be able to do something like:

 Tag.ProjectTags.Count();

Here is some additional code as requested:

my Tag Class:

 public class Tag
{
    public virtual IList<ProjectTag> ProjectTags { get; set; }
    public virtual string Name { get; set; }
    public virtual string Description { get; set; }
}

Answer 1

Somewhere in your code, you should have dereferenced the original collection on your Project domain. I suspect that your code goes like this:

var project = Session.Get<Project>();
project.ProjectsTags = new List<ProjectsTags> { someProjectsTagsInstance };
Session.Save(project);

If this is the case, you should do this instead:

var project = Session.Get<Project>();
project.ProjectsTags.Clear();
project.ProjectsTags.Add(someProjectsTagsInstance);
Session.Save(project);

Answer 2

While a collection is not modified, NH can still think that it is. Something like this could be caused by a ghost update. From NHibernate 3.0 Cookbook, Jason Dentler (page 184): "As part of automatic dirty checking, NHibernate compares the original state of an entity to its current state. An otherwise unchanged entity may be updated unnecessarily because a type conversion caused this comparison to fail".

Ghost update of collection can be caused by code that looks like this:

public class Tag
{
    private IList<ProjectTag> projectsTags;

    public virtual IEnumerable<ProjectTag> ProjectsTags
    {
        get
        {
            return new ReadOnlyCollection<ProjectTag>(projectsTags);
        }

        set
        {
            projectsTags = (IList<ProjectTag>)value;
        }
    }
}

ProjectsTags property returns the collection in readonly wrapper, so client code cannot add or remove elements to/from the collection.

The error will appear even when name of a tag is not changed:

private void GhostTagUpdate(int id)
{
    using (var session = OpenSession())
    {
        using (var transaction = session.BeginTransaction())
        {
            var tag = session.Get<Tag>(id);

            transaction.Commit();
        }
    }
}

ProjectsTags collection should be mapped with CamelCaseField access strategy to avoid ghost updated:

HasMany(x => x.ProjectsTags)
    .Access.CamelCaseField()
    .AsBag().Inverse().Cascade.AllDeleteOrphan().Fetch.Select().BatchSize(80);

Anyway...

Your association seems to be diabolically complex. If ProjectsTags table should contains only id of tag and id of project, then it would be simpler to use FNH many-to-many bidirectional mapping:

public class Tag2Map : ClassMap<Tag2>
{
    public Tag2Map()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        HasManyToMany(x => x.Projects)
            .AsBag()
            .Cascade.None()
            .Table("ProjectsTags")
            .ParentKeyColumn("TagId")
            .ChildKeyColumn("ProjectId");
    }
}

public class Project2Map : ClassMap<Project2>
{
    public Project2Map()
    {
        Id(x => x.Id);
        Map(x => x.Name);
        HasManyToMany(x => x.Tags)
            .AsBag()
            .Cascade.None()
            .Inverse()
            .Table("ProjectsTags")
            .ParentKeyColumn("ProjectId")
            .ChildKeyColumn("TagId");
    }
}

Now there is no need for ProjectTag entity in the model. The count of how many times is given tag used can be retrieved in two ways:

Direct way: tag.Projects.Count() - but it retrieves all projects from database.

Query way:

var tag = session.Get<Tag2>(tagId);
var count = session.Query<Project2>().Where(x => x.Tags.Contains(tag)).Count();