Controller Example

~/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.