Not null exception occurs during overwriting base class using EF Inheritance - entity-framework

Question

I are using TPT inheritance and the relationships of my entities is below:
DepartmentCatalog derived form base class catalog and contain another entity Department
My propurse is getting the name of DepartmentCatalog from Deparment not base class catalog and the name should be readonly. So I override the name like this:
using System;
using Rednoble.CMS.Domain.CatalogsEntity;
using Rednoble.CMS.Domain.ExpertsEntity;
using System.ComponentModel.DataAnnotations.Schema;
using Rednoble.CMS.Domain.DepartmentEntity;
namespace Rednoble.CMS.Domain.CatalogsEntity
{
[Table("DepartmentCatalog")]
public class DepartmentCatalog : Catalog
{
public DepartmentCatalog()
: base()
{
}
public Guid DepartmentId { get; set; }
public Department Department { get; set; }
public override string Name
{
get
{
return Department.Name;
}
}
}
}
But when I getting deparmentcatalog from context I got a not null exception because EF getting the name of DepartmentCatalog before setting the complex property - "Department".
I'm not sure it's the illegal design or it's the illegal using of TPT inheritance. How to get my work be done, can anyone help me?
BTW, I'm using VS2012 and EF 6.0.
** Update ***
I have tried Eagerly Loading and Explicitly Loading for hours:
https://msdn.microsoft.com/en-us/data/jj574232.aspx
var deptCatas = Context.Set<DepartmentCatalog>().Include(dc => dc.Department).Where(predicate);
deptCatas.Load();//throw not null exception
var departmentCatalogs = _departmentCatalogReposiory.FindBy(r => r.DepartmentId == id);
departmentCatalogs.FirstOrDefault();//throw not null exception
Here is my mapping:
public class CategoryMapping : EntityTypeConfiguration<Catalog>
{
public CategoryMapping()
{
Property(r => r.Name).IsRequired().HasMaxLength(150);
Property(r => r.PageUrl).HasMaxLength(500);
Property(r => r.EnglishName).HasMaxLength(100);
Property(r => r.CoverPictrue).HasMaxLength(500);
HasMany(r => r.Rala_Catalog_Articles).WithRequired(r => r.Catalog).HasForeignKey(r => r.CatalogId);
HasOptional(c => c.ViewTemplate).WithMany().HasForeignKey(r=>r.TemplateId).WillCascadeOnDelete(false);
}
}
public class DepartmentCategoryMapping
{
public DepartmentCategoryMapping()
{
HasRequired(dc => dc.Department).WithMany(d => d.DepartmentCatalogs).HasForeignKey(dc => dc.DepartmentId).WillCascadeOnDelete(true);
}
}
I've found out a way to solve this problem, just check the if Department is null when first time Call "FirstOrDefault" or "Load" for the property of name, if Department is null, return the name of base catalog. But I thought this is not good enough. one of the defect is when the name of department changed, DepartmentCatalog might get old value. Here is my code:
[Table("DepartmentCatalog")]
public class DepartmentCatalog : Catalog
{
public DepartmentCatalog()
: base()
{
}
public Guid DepartmentId { get; set; }
public Department Department { get; set; }
public override string Name
{
get
{
if (Department == null)
{
return base.Name;
}
else
{
return Department.Name;
}
}
}
}

Answer 1

because you didn't load Department.
You should Include Department in your query or use Lazy Loading.
** Update ***
for eager Loaing you can use same this code:
MyContext.DepartmentCatalogs.Include(x=> x.Department).ToList()
and for Lazy Loading you have to use virtual keyword for Department property.
BTW, for lazy loading, lazy loading and proxy must be true in your configuration.
**** EDIT ****
[ForeignKey("DepartmentId")]
public Department Department {get;set}
**** Update ****
context.DepartmentCatalogs.AsNoTracking().Include(x=>x.Department).FirstOrDefault();
the magic key to solving this problem is AsNoTracking.