db.savechanges and db.dispose conflict removes my newly created object reference

In my controller i am creating a new subcategory object and saving it to my database like this:

    [Authorize(Roles = "administrator")]
     [HttpPost]
     public ActionResult Create(CategoryViewModel viewmodel, HttpPostedFileBase Icon)
     {
         SubCategory subcategory = viewmodel.subcategory;
 
         subcategory.Category = categorycontroller.getCategoryByName(viewmodel.SelectedValue);
 
         if (Icon != null && Icon.ContentLength > 0)
         {
             // extract only the filename
             var fileName = Path.GetFileName(Icon.FileName);
             // store the file inside ~/App_Data/uploads folder
             var path = Path.Combine(Server.MapPath("../../Content/icons/"), fileName);
             Icon.SaveAs(path);
             subcategory.Icon = fileName;
         }
 
         if (ModelState.IsValid)
         {
             db.subcategories.Add(subcategory);
             db.SaveChanges();
             return RedirectToAction("Index");  
         }
 
         return View(subcategory);
     }
 

After debugging the application i noticed that the controller correctly saves all my data including the reference to a category object in my newly created subcategory.

problem is when i load the data from my db later on the subcategory object is missing its reference to my category object.

my viewmodel looks like this:

using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Web;
 using System.Data.Entity;
 using System.Web.Mvc;
 using SkyLearn.Areas.Categories.Controllers;
 
 namespace SkyLearn.Areas.Categories.Models
 {
     public class CategoryViewModel
     {
         public List<SelectListItem> PossibleValues { get; set; }
         public string SelectedValue { get; set; }
         public SubCategory subcategory { get; set; }
 
         public CategoryController categorycontroller;
 
         public CategoryViewModel()
         {
             PossibleValues = new List<SelectListItem>();
         }
     }
 }
 

and my method on the category controller that finds the object:

public Category getCategoryByName(string categoryname)
 {
     foreach (Category cat in getCategories())
     {
         if (cat.Title == categoryname)
         {
             return cat;
         }
     }
     return null;
 }
 

why does my category object reference disappear guys? im in a blind

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