form objects

post_form.rb

class PostForm
  include Virtus.model
  extend ActiveModel::Naming
  include ActiveModel::Validations

  attribute :id, Integer
  attribute :title, String
  attribute :body, String

  validates :title, presence: true

  def persisted?
    id.present?
  end

  def self.model_name
    @_model_name ||= ActiveModel::Name.new(self, nil, "Post")
  end

  def initialize(attributes, id = nil)
    super(attributes)
    self.id = id if id # force id from params[:id] if present
  end

  def save
    if valid?
      # save all the things here
    end
  end

  def fetch_post
    persisted? ? Post.find(id) : Post.new
  end
end

posts_controller.rb

class PostsController
 def edit
    @post = PostForm.new(post.attributes, post.id)
  end

  def update
    @post = PostForm.new(params[:post], params[:id])

    if @post.save
      redirect_to @post
    else
      render :edit
    end
  end

  private

  def post
    Post.find(params[:id])
  end
end