Nested resources create Couldn't find Topic without an ID - ruby-on-rails-3

Question

I wanna make an application which User can create a topic and others can make posts after that. I nested my resources in my routes.rb:
MyPedia2::Application.routes.draw do
resources :users
resources :sessions, only: [:new, :create, :destroy]
resources :topics, only: [:show, :create, :destroy]
resources :posts
resources :topics do
resources :posts, only: [:create, :show, :new]
end
In my topic show page , I want to show topic.title and sended Posts and post.form.html.erb.
Everything works accept when i create a post , I get mistake
ActiveRecord::RecordNotFound in PostsController#create
Couldn't find Topic without an ID..
This is my posts_controller.rb:
class PostsController < ApplicationController
before_filter :signed_in_user, only: [:create, :destroy]
before_filter :correct_user, only: :destroy
def new
#topic= Topic.find_by_id(params[:id])
#post = #topic.posts.build(params[:post])
end
def show
#topic = Topic.find(params[:id])
#posts = #topic.posts.paginate(page: params[:page])
end
def create
#topic = Topic.find(params[:id])
#post = #topic.posts.build(params[:post])
#post.topic = #topic
if #post.save
flash[:success] = "Konu oluşturuldu!"
redirect_to :back
else
render 'static_pages/home'
end
end
def destroy
#post.destroy
redirect_to root_path
end
private
def correct_user
#post = current_user.posts.find_by_id(params[:id])
redirect_to root_path if #post.nil?
end
end
and _post_form.html.erb:
<%= form_for #new_post do |f| %>
<%= render 'shared/error_messages', object: f.object %>
<div class="field">
<%= f.text_area :content, placeholder: "yorumunuzu girin..." %>
</div>
<%= f.submit "Gönder", class: "btn btn-large btn-primary" %>
<% end %>

Answer 1

There are a few things that should solve things for you.
First and foremost, your create action in the posts controller is a bit wrong - it should look something like this:
def create
#topic = Topic.find(params[:topic_id])
#post = #topic.posts.build(params[:post])
# This is unnecessary as you're already adding
# the post to the topic with the build statement.
# #post.topic = #topic
if #post.save
flash[:success] = "Konu oluşturuldu!"
redirect_to :back
else
render 'static_pages/home'
end
end
This controller action assumes that you're going to use a put request to a post resource that is nested in topics, so you'll have to clean up your routes.
You have routes to posts#create both nested and unnested.
If posts are ALWAYS supposed to be nested within a topic, which your controller logic is stating, then you should add this to the unnested posts resource:
resources :posts, except: [:new, :create]
and then change that form_for tag to this:
<%= form_for [#topic, #post] do |f| %>
This tells the form builder that you're using a nested resource and will use the correct url for the http request.
Also - it looks like you're using loading all of your topics using Topic.find(params[:id]). This isn't going to work - you're in the posts controller, this is a post id. You should be loading posts with the id param like this: Post.find(params[:id]) and then the topic like this: topic = post.topic