~/Models/BlogPost.cs:

public class BlogPost
{
	public int BlogPostID { get; set; }
	public string Title { get; set; }
	public string Content { get; set; }
	public DateTime LastEdited { get; set; }
	public ApplicationUser Author { get; set; }
}

~/Models/ApplicationUser.cs

public class ApplicationUser : IdentityUser
{
	public string Nickname;
	public int Age;
}

~/Controllers/BlogController.cs:

private readonly ApplicationDbContext _context;

public async Task<IActionResult> Edit(int id, [Bind("BlogPostID, Title, Content")] BlogPost post)
{
	var author = _context.BlogPosts
					.Include(entry => entry.Author)
					.AsNoTracking()
					.FirstOrDefaultAsync(entry => entry.BlogPostId == id)
					.Result
					.Author;
	
	// Users should only be able to edit their own posts
	if(author.Id != User.Id)
	{
		return RedirectToAction(nameof(Index));
	}

	// Do some operations like updating when the post was edited
	post.LastEdited = DateTime.Now;

	_context.Update(post);
	await _context.SaveChangesAsync();
}

// But what if we have abstracted retrieving posts to a service?

~/Services/BlogPostService.cs:

private readonly ApplicationDbContext _context;

public async Task<BlogPost> GetBlogPostAsync(int id)
{
	var post = _context.BlogPosts
					.Include(entry => entry.Author)
					.FirstOrDefaultAsync(entry => entry.BlogPostId == id)
					.Result;

	// Verify that the post exists etc. here

	return post;
}

// Then we get the following problem in the controller:

~/Controllers/BlogController.cs:

private readonly IBlogPostService _blogPostService;

public async Task<IActionResult> Edit(int id, [Bind("BlogPostID, Title, Content")] BlogPost post)
{
	// This throws an error because there are now two instances tracking the same Primary Key { BlogPostID }
	var author = _blogPostService.GetBlogPostAsync(id).Result.Author;
	
	// Users should only be able to edit their own posts
	if(author.Id != User.Id)
	{
		return RedirectToAction(nameof(Index));
	}

	// Do some operations like updating when the post was edited
	post.LastEdited = DateTime.Now;

	_context.Update(post);
	await _context.SaveChangesAsync();
}

// If I modify the service class to use .AsNoTracking() when grabbing the post it could cause issues in situations where I DO want to track the changes.

// I could always have a function GetBlogPostAsyncAsNoTracking(int id); but that seems excessive and would cause redundant code. Furthermore, using an optional boolean argument to the GetBlogPostAsync(int id, bool? AsNoTracking); could work, but seems like a bad idea as well.