Entity framework doesn't save object property

Question

I'm developing online store, using ASP.Net MVC 4 framework.

I have an object Category — category of items in store (men's apparel — for instance):

public class Category
{
    [Required]
    public long id { get; set; }

    [Required]
    public string name { get; set; }

    public bool active { get; set; }
    public bool displayOnTop { get; set; }
    public bool showSubcategoriesItems { get; set; }

    public Category parentCategory { get; set; }

    public virtual Collection<Item> items { get; set; }

    public Category()
    {
        this.items = new Collection<Item>();
    }
}

There is a property parentCategory — which means that this category is included to another one (Men's apparel > Shirts, for instance).

For editing existing Category I use the following action:

[HttpPost]
public ActionResult EditCategory(Category editedCategory, string parentCategorySelector)
{
    UsersContext db = new UsersContext();

    Category existingCategory = db.categories.SingleOrDefault(c => c.id == editedCategory.id);
    long parentCategoryId = (long)Int32.Parse(parentCategorySelector);
    Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId);

    if (existingCategory == null) return RedirectToAction("Index", "Admin");

    existingCategory.name = editedCategory.name;
    existingCategory.active = editedCategory.active;
    existingCategory.displayOnTop = editedCategory.displayOnTop;
    existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems;
    existingCategory.parentCategory = parentCategory;

    db.Entry(existingCategory.parentCategory).State = EntityState.Modified;
    db.SaveChanges();

    return View(existingCategory);
}

Everything works fine, except when I want to set parentCategory to null — it sets to null in code, but null value is not being saved to database — previous value still there.

But if I do it like this:

[HttpPost]
public ActionResult EditCategory(Category editedCategory, string parentCategorySelector)
{
    UsersContext db = new UsersContext();

    Category existingCategory = db.categories.SingleOrDefault(c => c.id == editedCategory.id);
    long parentCategoryId = (long)Int32.Parse(parentCategorySelector);
    Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId);

    if (existingCategory == null) return RedirectToAction("Index", "Admin");

    existingCategory.name = editedCategory.name;
    existingCategory.active = editedCategory.active;
    existingCategory.displayOnTop = editedCategory.displayOnTop;
    existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems;
    existingCategory.parentCategory = null;

    db.Entry(existingCategory.parentCategory).State = EntityState.Modified;
    db.SaveChanges();

    return View(existingCategory);
}

i.e. I set it to null without conditions, it is being saved correctly.

What am I doing wrong?

Answer 1

Why there is no Key attribute under Category.id? If the id is a primary key it must be there.

You should use Include to load relate entities:

   [HttpPost]
    public ActionResult EditCategory(Category editedCategory, string parentCategorySelector)
    {
        using(UsersContext db = new UsersContext())
        {
            Category existingCategory = db.categories.Include(c => c.parentCategory).SingleOrDefault(c => c.id == editedCategory.id);
            long parentCategoryId = (long)Int32.Parse(parentCategorySelector);
            Category parentCategory = db.categories.SingleOrDefault(c => c.id == parentCategoryId);

            if (existingCategory == null) return RedirectToAction("Index", "Admin");

            existingCategory.name = editedCategory.name;
            existingCategory.active = editedCategory.active;
            existingCategory.displayOnTop = editedCategory.displayOnTop;
            existingCategory.showSubcategoriesItems = editedCategory.showSubcategoriesItems;
            existingCategory.parentCategory = parentCategory;

            db.SaveChanges();

            return View(existingCategory);
        }
    }

Please note, how DbContext is properly disposed.