routes.rb у вас что?resources :posts do
resources :comments
endresources :posts
resources :commentsquestion/add_commentclass QuestionsController < ApplicationController
def add_comment
@question = Question.find(params[:id])
@answer = Answer.new
@answer.question = @question
@answer.user = current_user
@answer.body = params[:answer][:body]
@answer.parrent_id = params[:answer_id] #из hidden поля
if @answer.save
flash[:notice] = "Answer was successfully created..."
redirect_to question_path(@question)
else
flash.now[:error] = "Answer validation error ..."
render "questions/show"
end
end
endroutes.rb и виденья что передаётся на сервак, трудно помочь.