entity framework - Problems using TPT (Table Per Type) in EF 4.2 and deletion of parent objects

Question

From what I understand on several posts the TPT architecure, with EF, does not create the necessary ON DELETE CASCADE when using a shared primary key.... It was also said that the EF context will handle the proper order of deletion of the sub-classed tables (however I do get an error that it breaks the constraint and that I can fix it with adding the ON DELETE CASCADE on the sub-class table)...

more background info...

I have a Section class, which has a number, title, and a list of pages. The page is designed using a super class which holds basic page properties. I have about 10+ sub-classes of the page class. The Section class holds an ICollection of these pages. The DB is created properly with the exception of no ON DELETE CASCADE on the sub-classed tables.

My code will create the entities and adds to the DB fine. However, if I try to delete a section (or all sections) it fails todelete due to the FK constraint on my sub-class page table...

public abstract BaseContent 
{
... common properties which are Ignored in the DB ...
}

public class Course : BaseContent
{
    public int Id {get;set;}
    public string Name {get;set;}
    public string Descripiton {get;set;}
    public virtual ICollection<Chapter> Chapters{get;set;}
    ...
}

public class Chapter : BaseContent
{
    public int Id {get;set;}
    public int Number {get;set;}
    public string Title {get;set;}
    public virtual Course MyCourse{get;set;}
    public virtual ICollection<Section> Sections{get;set;}
    ...
}

public class Section : BaseContent
{
    public int Id {get;set;}
    public int Number {get;set;}
    public string Title {get;set;}
    public virtual Chapter MyChapter {get;set;}
    public virtual ICollection<BasePage> Pages {get;set;}
    ...
}

public abstract class BasePage : BaseContent, IComparable
{
    public int Id { get; set; }
    public string Title { get; set; }
    public string PageImageRef { get; set; }
    public ePageImageLocation ImageLocationOnPage { get; set; }
    public int PageNumber { get; set; }
    public virtual Section MySection { get; set; }
    ...
}

public class ChapterPage : BasePage
{
    public virtual int ChapterNumber { get; set; }
    public virtual string ChapterTitle  { get; set; }
    public virtual string AudioRef { get; set; }
}

public class SectionPage : BasePage
{
    public virtual int SectionNumber { get; set; }
    public virtual string SectionTitle  { get; set; }
    public virtual string SectionIntroduction { get; set; }
}

... plus about 8 other BasePage sub-classes...

public class MyContext: DbContext
{
...
    public DbSet<Course> Courses { get; set; }
    public DbSet<Chapter> Chapters { get; set; }
    public DbSet<Section> Sections { get; set; }
    public DbSet<BasePage> Pages { get; set; }
...
}

.. Fluent API ... (note Schema is defined to "" for SqlServer, for Oracle its the schema name)

private EntityTypeConfiguration<T> configureTablePerType<T>(string tableName) where T : BaseContent
{
    var config = new EntityTypeConfiguration<T>();

    config.ToTable(tableName, Schema);

    // This adds the appropriate Ignore calls on config for the base class BaseContent
    DataAccessUtilityClass.IgnoreAllBaseContentProperties<T>(config);

    return config;
}

public virtual EntityTypeConfiguration<BasePage> ConfigurePageContent()
{
    var config = configureTablePerType<BasePage>("PageContent");

    config.HasKey(pg => pg.Id);
    config.HasRequired(pg => pg.Title);
    config.HasOptional(pg => pg.PageImageRef);

    config.Ignore(pg => pg.ImageLocationOnPage);

    return config;
}

public virtual EntityTypeConfiguration<ChapterPage> ConfigureChapterPage()
{
    var config = configureTablePerType<ChapterPage>("ChapterPage");

    config.HasOptional(pg => pg.AudioRef);
    config.Ignore(pg => pg.ChapterNumber);
    config.Ignore(pg => pg.ChapterTitle);

    return config;
}

public virtual EntityTypeConfiguration<SectionPage> ConfigureSectionPage()
{
    var config = configureTablePerType<SectionPage>("SectionPage");

    config.HasOptional(pg => pg.AudioRef);
    config.Ignore(pg => pg.SectionNumber);
    config.Ignore(pg => pg.SectionTitle);

    return config;
}

... other code to model other tables...

So the app is able to populate content and the relationships are properly set up. However, when I try to delete the course, I get the error that the delete failed due to the constraint on the ChapterPage to PageContent table..

Here is the code which deletes the Course (actually I delete all courses)...

using (MyContext ctx = new MyContext())
{
    ctx.Courses.ToList().ForEach(crs => ctx.Courses.Remove(crs));
    AttachLookupEntities(ctx);
    ctx.SaveChanges();
}

If I add the 'ON DELETE CASCADE' in the ChapterPage and SectionPage table for its shared primary with PageContent, the delete goes through.

In summary,

The only solution that I have seen is to manually alter the constraints to add the ON DELETE CASCADE for all of my sub-class page tables. I can implement the change, as I have code which generates the DB script for the EF tables I need (a small subset of our whole DB) since we will not use EF to create or instantiate the DB (since it does not properly support migrations as yet...).

I sincerely hope that I have miscoded something, or forgot some setting in the model builder logic. Because if not, the EF designers have defined an architecure (TPT design approach) which cannot be used in any real world situation without a hack workaround. It's a half finished solution. Do not get me wrong, I like the work that has been done, and like most MSFT solutions its works for 70% of most basic application usages. It just is not ready for more complex situations.

I was trying to keep the DB design all within the EF fluent API and self-contained. It's about 98% there for me, just would be nice if they finished the job, maybe in the next release. At least it saves me all the CRUD operations.

Ciao! Jim Shaw

Answer 1

I have reproduced the problem with a little bit simpler example:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Data.Entity;

namespace EFTPT
{
    public class Parent
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public ICollection<BasePage> Pages { get; set; }
    }

    public abstract class BasePage
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public Parent Parent { get; set; }
    }

    public class DerivedPage : BasePage
    {
        public string DerivedName { get; set; }
    }

    public class MyContext : DbContext
    {
        public DbSet<Parent> Parents { get; set; }
        public DbSet<BasePage> BasePages { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Parent>()
                .HasMany(p => p.Pages)
                .WithRequired(p => p.Parent);  // creates casc. delete in DB

            modelBuilder.Entity<BasePage>()
                .ToTable("BasePages");

            modelBuilder.Entity<DerivedPage>()
                .ToTable("DerivedPages");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            using (var ctx = new MyContext())
            {
                var parent = new Parent { Pages = new List<BasePage>() };
                var derivedPage = new DerivedPage();

                parent.Pages.Add(derivedPage);

                ctx.Parents.Add(parent);
                ctx.SaveChanges();
            }

            using (var ctx = new MyContext())
            {
                var parent = ctx.Parents.FirstOrDefault();
                ctx.Parents.Remove(parent);
                ctx.SaveChanges();  // exception here
            }
        }
    }
}

This gives the same exception that you had too. Only solutions seem to be:

• Either setup cascading delete for the TPT constraint in the DB manually, as you already tested (or put an appropriate SQL command into the Seed method).

• Or load the entites which are involved in the TPT inheritance into memory. In my example code:

  var parent = ctx.Parents.Include(p => p.Pages).FirstOrDefault();
  

  When the entities are loaded into the context, EF creates actually two DELETE statements - one for the base table and one for the derived table. In your case, this is a terrible solution because you had to load a much more complex object graph before you can get the TPT entities.

Even more problematic is if Parent has an ICollection<DerivedPage> (and the inverse Parent property is in DerivedPage then):

public class Parent
{
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<DerivedPage> Pages { get; set; }
}

public abstract class BasePage
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class DerivedPage : BasePage
{
    public string DerivedName { get; set; }
    public Parent Parent { get; set; }
}

The example code wouldn't throw an exception but instead delete the row from the derived table but not from the base table, leaving a phantom row which cannot represent an entity anymore because BasePage is abstract. This problem is not solvable by a cascading delete but you were actually forced to load the collection into the context before you can delete the parent to avoid such a nonsense in the database.

A similar question and analysis was here: http://social.msdn.microsoft.com/Forums/en-US/adodotnetentityframework/thread/3c27d761-4d0a-4704-85f3-8566fa37d14e/