// Post class includes
 public Post()
        {
            Themes = new List<Theme>();
        }
        public virtual ICollection<Theme> Themes { get; set; }
// Themes class

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

//A post view model brings in the information on themes to my business layer.
//In my business layer.
 Post post = new Post
            {
                Header = postViewModel.Header,
                CreatedDate = DateTime.Now,
            };
     foreach (var themeModel in postViewModel.ThemeModels)
            {
                if (themeModel.isSelected)
                {
                    var theme = new Theme{ Id = themeModel.ThemeId  };
                    RepositoryFactory.AttachEntity(theme);
                    post.Themes.Add(theme);
                }
            }
 RepositoryFactory.SaveEntity(post);
// In my data layer
        public void AttachEntity<TEntity>(TEntity entity) where TEntity : Entity
        {
            using (var context = new MyContext())
            {
                context.Set<TEntity>().Attach(entity);
// I have tried putting context.SaveChanges(); here too but does not seem to matter
            }
        }
public void SaveEntity<TEntity>(TEntity entity) where TEntity : Entity
        {
            using (var context = new MyContext())
            {
                if (entity.Id == 0)
                {
                    context.Set<TEntity>().Add(entity);
                }
                else
                {
                    context.Entry(entity).State = EntityState.Modified;
                }

                context.SaveChanges();
            }
        }