Prefill form from associated object on error in Rails

I have a Post that has_many :comments and a Comment that belongs_to :post. On /posts/:id (the post show method开发者_运维技巧) I render a form where users can leave comments.

It all works, validations, tests and posting is just fine. Only thing missing is how to re-render the POSTed data on validation errors.

The (simplified) code for this is:

#app/controllers/posts_controller.rb
class PostsController < ApplicationController
  def index
    @posts = Post.all_published(params[:page])
    @title = "Blog"
  end

  def show
    @post = Post.where({:published => true}).find(params[:id])
    @comment = Comment.new(:post => @post)
    @title = @post.title
  end
end

#app/controllers/comments_controller.rb
class CommentsController < ApplicationController
  def create
    @comment = Comment.new(params[:comment])
puts @comment

    if @comment.save
      flash[:notice] = 'Comment was successfully created.'
      redirect_to(@comment.post)
    else
      flash[:notice] = "Error creating comment: #{@comment.errors}"
      redirect_to(@comment.post)
    end
  end
end

#app/views/posts/show.haml
.html renders Post contents.

- form_for @comment do |f|
  = f.hidden_field :post_id
  = f.text_area :body
  = f.text_field :name
  .some more fields.

I expect the solution to be either in some magical declaration in the comments_controller.rb, part

   else
      flash[:notice] = "Error creating comment: #{@comment.errors}"
      redirect_to(@comment.post)
    end

Or in the PostsController.show where I prepare the @comment. Should I set that @comment conditional and fill it with some magic variable on errors? Or did I make some entirely different mistake?

If you redirect, that data is usually lost, thats why in most cases in create create actions you would have noticed that in the false scenario, render not redirect_to.

So instead you could just try,

 flash[:notice] = ""Error creating comment: #{@comment.errors}"
 render :template => "posts/show"
 @post = @comment.post
 # you may need to pre-populate the instance variables used inside PostsController#show