How can I resolve a validation error related to the binding of a related entity (Category) in ASP.NET Core 6 MVC, despite trying various approaches?

Hello, I'm relatively new to ASP.NET Core 6 and I'm encountering a frustrating validation issue when attempting to work with related entities in my MVC application. I apologize if this question has been asked before, but I've spent considerable time trying to solve it without success, and I'm hoping for some guidance. In my application, I have two classes, Expense and Category, with a one-to-many relationship between them. My goal is to add a new Expense through a form submission, but every time I try, I face validation errors specifically related to the Category field. Here's a simplified version of my code: // Expense.cs

public class Expense { 
	public int Id { get; set; }
	// Other properties...

	[Required(ErrorMessage = "The Category field is required.")]
	public Category Category { get; set; }

	public int CategoryId { get; set; }
}

// ExpenseController.cs

[HttpPost] [ValidateAntiForgeryToken] public IActionResult AddExpense([Bind("Id,Description,Date,Amount,CategoryId")] Expense expense) { 
    if (!ModelState.IsValid) 
    { 
      return ValidationProblem(ModelState); 
    }
    _context.Add(expense);
    _context.SaveChanges();
    return RedirectToAction("Index");

}

<div class="form-group"><label asp-for="expense.CategoryId" class="control-label">
    </label><input type="hidden" asp-for="expense.Category" />
    <select asp-for="expense.CategoryId" asp-items="@(new SelectList(Model.Categories, "Id", "Type"))" class="form-control">
        <option value="">Select a category</option>
    </select>
    <span asp-validation-for="expense.CategoryId" class="text-danger"></span>
</div>

Despite setting up the relationship and configuring the form correctly, I keep getting validation errors stating that the Category field is required. I've tried removing the [BindProperty] attribute and ensuring that the foreign key (CategoryId) is correctly set, but the issue persists. Could someone please advise on how to properly bind the Category property in the post-action to avoid these validation errors? Any help would be greatly appreciated. Thank you!