Entity Framework Cascading Delete

Question

First of all, apologies if I\'m missing some basic stuff here but I\'m new to EF and still getting my head around setting up the DB code first....

I\'m having a

Answer 1

You need to disable cascade deletes for all but one of the possible paths. In your case you have the following paths:

Project -> UseCase
Project -> Actor -> UseCase

You can allow a single path for cascading deletion of UseCase - via the Project entity or Actor entity. However, if we disable cascading deletes in the Project -> UseCase path, we'll still achieve a cascading delete via Actor:

modelBuilder.Entity<Project>()
    .HasMany( p => p.UseCases )
    .WithRequired( uc => uc.Project )
    .HasForeignKey( uc => uc.ProjectID )
    .WillCascadeOnDelete( false ); // disable this cascading delete

modelBuilder.Entity<Project>()
    .HasMany( p => p.Actors )
    .WithRequired( a => a.Project )
    .HasForeignKey( a => a.ProjectID )
    .WillCascadeOnDelete( true ); // this works

modelBuilder.Entity<Actor>()
    .HasMany( a => a.UseCases )
    .WithRequired( uc => uc.Actor )
    .HasForeignKey( uc => uc.ActorID )
    .WillCascadeOnDelete( true ); // and this works

Side note:

Your model has a data inconsistency hazard - both Actor and UseCase have a FK to Project via ProjectID, but there is nothing in the model to enforce the Actor referenced by a UseCase has the same ProjectID - an Actor from "Project 1" could be reference by a UseCase from "Project 2". You could include the ProjectID in the Actor PK and then in the UseCase->Actor FK, ensuring that the Actor referenced by a UseCase belongs to the same Project, but this would technically violate the 2NF.

The 'proper' model is probably a Project->Actors->UseCases hierarchy, simply requiring you to join through Actors to get a Project's UseCases

Answer 2

You need to make ActorID in your UseCase class as a nullable int. EF is throwing that error because it sees 2 foreign keys that are required in a single class. Having that would create multiple cascade paths--something that SQL Server is, unfortunately, ill-equipped to handle.

In any case, making Actor optional on your UseCase class will mean that the Actor won't be deleted when the UseCase is, which I believe is your intent.