Unable to delete child entities from a POCO using Unit Of Work pattern

Question

I am using POCO classes on an EF4 CTP5 project and I am having trouble deleting child properties. Here's my example (hopefully not too long).

Relevant Portions of the Tour Class

public partial class Tour
{
  public Guid TourId { get; private set; }
  protected virtual List<Agent> _agents { get; set; }

  public void AddAgent(Agent agent)
  {
    _agents.Add(agent);
  }

  public void RemoveAgent(Guid agentId)
  {
    var a = Agents.Single(x => x.AgentId == agentId);
    _agents.Remove(Agents.Single(x => x.AgentId == agentId));
  }
}

Command Handler

public class DeleteAgentCommandHandler : ICommandHandler<DeleteAgentCommand>
{
  private readonly IRepository<Core.Domain.Tour> _repository;
  private readonly IUnitOfWork _unitOfWork;

  public DeleteAgentCommandHandler(
      IRepository<Core.Domain.Tour> repository, 
      IUnitOfWork unitOfWork
    )
  {
    _repository = repository;
    _unitOfWork = unitOfWork;
  }

  public void Receive(DeleteAgentCommand command)
  {
    var tour = _repository.GetById(command.TourId);
    tour.RemoveAgent(command.AgentId);

    // The following line just ends up calling
    // DbContext.SaveChanges(); on the current context.

    _unitOfWork.Commit();
  }
}

Here's the error that I get when my UnitOfWork calls DbContext.SaveChanges()

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

This is happening because EF wont just automatically delete the an Agent entity from the database just because it has been removed from the Agents collection in my Tour class.

I need to explicitly call dbContext.Agents.DeleteObject(a);, but my problem is, I don't have access to the dbContext from within my POCO.

Is there any way to handle this scenario?

I think you're on the right path. It appears EF is trying to remove an Agent entity from the collection but it cannot set its TourId foreign key value to null (assuming there is a TourId foreign key on Agent.) How is the Tour-Agent association declared in the edmx? — kdawg
I am declaring the relationship with the Fluent API (4.1 codefirst) in OnModelCreating like so modelBuilder.Entity<Agent>().HasRequired(p => p.Tour).WithMany(t => t.Agents); — jessegavin

Slauma Slauma · Accepted Answer · 2011-03-17T20:26:25

With your current architecture I am afraid you need to feed your DeleteAgentCommandHandler with a second repository (IRepository<Core.Domain.Agent>, I guess) and then call something like Delete(command.AgentId) on that second repository.

Or you could extend your IUnitOfWork to be a factory of repositories, so the interface would get an additional method like T CreateRepository<T>() which allows you to pull any instance of your generic repository from the unit of work. (Then you only need to inject IUnitOfWork into the DeleteAgentCommandHandler, and not the repositories anymore.)

Or stay away from generic repositories in your business/UI layer. If Agent is completely dependent on Tour it doesn't need to have a repository at all. A non-generic ITourRepository could have methods to handle the case of removing an agent from a tour in the database layer appropriately.

Unable to delete child entities from a POCO using Unit Of Work pattern

2 Answers