Topic: rendring simple_form errors problem

I have list with posts (pages#home), on click I open list with comments (posts#show), where I declare variable @feed_items. In the end of the comments I have simple_form for new comment (comments#create). Problem: if error occuring on submit button, I need to render existed list with comments and form with errors.
I'm trying in comments#create:
    if @comment.save
      ...
    else
      render 'posts/show'
    end
but in this case variable @feed_items isn't declared because method posts#show didn't called. When I trying to write redirect_to I see list with comments but without error messages. How to do this?