Have a create form at index across models

I want to ask if what I have done is a good practice.

I have two models: a discussion has many posts.

In my discussions#show all posts are displayed. Now I want to add a form, so I can create a new post at that discussion.

discussions_controller.rb show()

@discussion = Discussion.find(params[:id])
@post = @discussion.posts.build

show.html.erb

<%= render :partial => 'posts/form' %>
<%= render @discussion.开发者_开发技巧posts %>
<%= link_to 'Edit', edit_discussion_path(@discussion) %> |
<%= link_to 'Back', discussions_path %>

_post.html.erb

<div class="post">    
  <p>
    <b>Content:</b>
    <%= post.content %>
  </p>
  <%= link_to 'Edit', edit_post_path(post) %> |
  <%= link_to 'Back', posts_path %>
</div>

This will cause an error at the 'edit' and 'back' link_to methods, because the new post object I built will become part of the @discussion.posts, and since that new post is not saved, the edit_post_path url helper will give out "No route matches" error(it can't find that unsaved post).

If I want to keep 'edit' and 'back' link_to methods, I have to filter out the newly built (but not saved) object. But somehow I feel that what I have done may be hackish, and want to ask if there is any better practice to achieve my goal.

If you don't need the discussion_id when submitting the new Post form, then you can simply do

@post = Post.new

If you do need discussion_id set on the new Post object then you can do that without adding the new object to @discussion.posts

@post = Post.new(:discussion => @discussion)