MVC Linq查询返回继承的实体的ID，但是该实体为null。 我如何.Include（）TPH成员？

Question

I have an MVC C# application using Razor Views. I have just implemented Table-Per-Hierarchy (TPH) and have run into an issue where I cannot .Include() the inherited Entity.

Project Details View:

@Html.DisplayFor(model => model.FundingOpportunity.FundingProgram.FundingAgency.Name)

When I attempt to show the contents of FundingAgency, it is always null. I can also modify the controller to .include FundingAgency, but it still comes up null.

Modified Project Controller:

    // GET: /Project/Details/5
    public ActionResult Details(int? id)
    {
        if (id == null)
        {
            return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
        }

        var projectList = db
                            .Projects
                            .Include(p => p.FundingOpportunity)
                            .Include(p => p.FundingOpportunity.FundingProgram)
                            .Include(p => p.FundingOpportunity.FundingProgram.FundingAgency);

        Project project = projectList.Where(p => p.ID == id).First();

        if (project == null)
        {
            return HttpNotFound();
        }

        return View(project);
    }

Here I have modified the Project Controller to .Include the FundingAgency. You can see how the FundingAgencyID has a value and that value exists in the database, but the link to the FundingAgency Entity is not there. This is only happening with Table-Per-Hierarchy (TPH) items.

Answer 1

You can try as shown below.

Program program = db.Programs.Where(X => X.ID == id)
                          .Include(p => p.Agency)
                          .First();

Answer 2

I think the problem is in this line:

public class Agency : Contact
{
 -->   public virtual ICollection<Program> Programs { get; set; }
}

It should be:

public virtual ICollection<FundingProgram> Programs {get; set;}

In addition to this, this line of code might be wrong:

Program program = programs.Where(X => X.ID == id).First();

if (project == null) // Where does your project come from? Did you mean program?
{
    return HttpNotFound();
}

Answer 3

I will post my solution for those searching in the future for a similar issue.

Update:

During the Table-Per-Hierarchy (TPH) migration, the seed/migration file order was moved and this caused the SQL query to return the correct row, then check the Discriminator and that check failed, so it returned null. This was deceptive because when you step through the process, the ID was not null and that row on the table contained data.

Solution:

I updated the seed files so that the correct ID range was attached to FundingOpportunity.