Untitled

No data is saved in the DB and no errors is fired
public class Post
{
    [Key]
    public int PostID { get; set; }
    ...
    public virtual ICollection<Tag> Tags { get; set; }
}

public class Tag
{
    [Key]
    public int TagID { get; set; }

    [Required, StringLength(50)]
    public string Name { get; set; }

    public virtual ICollection<Post> Posts { get; set; }
}

Tag t = m_TagRepository.GetTag(tag.Trim().ToUpper());
    if (t == null) t = new Tag { Name = tag.Trim().ToUpper() };
    post.Tags.Add(t);

public void SavePost(Post post)
    {
        if (post.PostID == 0)
        {
            m_Context.Posts.Add(post);
        }
        else
        {
            var entry = m_Context.Entry(post);
            entry.State = EntityState.Modified;
        }
        m_Context.SaveChanges();
    }

[Authorize, HttpPost, ValidateInput(false), Theme("Admin")]
    public ActionResult Edit(PostFullViewModel postToEdit)
    {
        if (!ModelState.IsValid)
            return View();

        Post post = Mapper.Map<PostFullViewModel, Post>(postToEdit);
        m_PostBusiness.UpdateTags(post, postToEdit.TagString);
        m_PostBusiness.SavePost(post);
        TempData.SetStatusMessage(Strings.Post_SavedSuccessfully);

        return RedirectToAction("Manage");
    }

var post = GetPostFromYourRequest();
context.Posts.Attach(post);
ProcessTags(post, postToEdit);
context.SaveChanges();