Polymorphic association: NoMethodError - ruby-on-rails

Question

NoMethodError in PostsController#create
undefined method `posts' for nil:NilClass
Posts#controller
class PostsController < ApplicationController
# before_action :logged_in_user, only: [:create, :destroy]
before_action :correct_user, only: :destroy
before_action :find_postable, except: [:index]
def index
#posts = Post.all.order('created_at DESC')
end
def new
#post = Post.new
end
def create
#post = #postable.posts.new post_params
if #post.save
redirect_to :back, notice: 'Your comment was successfully posted!'
else
redirect_to :back, notice: "Your comment wasn't posted!"
end
end
def destroy
Post.find(params[:id]).destroy
redirect_to posts_path#, notice: "Post successfully deleted"
end
def edit
#post = Post.find(params[:id])
end
def update
#post = Post.find(params[:id])
if #post.update_attributes(student_params)
redirect_to post_path
else
redirect_to edit_post_path
end
end
def show
#post = Post.find (params[:id])
end
private
def post_params
params.require(:post).permit(:content, :image, :document, :audio, :video, :classroom_id)
end
def correct_user
#post = current_user.posts.find_by(id: params[:id])
redirect_to root_url if #post.nil?
end
def find_postable
#postable = Post.find_by_id(params[:post_id]) if params[:post_id]
#postable = Classroom.find_by_id(params[:classroom_id]) if params[:classroom_id]
end
Here is the post model
class Post < ActiveRecord::Base
belongs_to :user
belongs_to :classroom
belongs_to :postable, polymorphic: true
has_many :posts, as: :postable
end
Here is the classroom model (it belongs to the class and this is selected in the new post form"
class Classroom < ActiveRecord::Base
has_many :posts, as: :postable
belongs_to :teachers
# belongs_to :students
Posts in Schema.rb
create_table "posts", force: :cascade do |t|
t.text "content"
t.integer "user_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
t.integer "classroom_id"
t.integer "postable_id"
t.string "postable_type"
end
I've tried many things with no success, I'm not quite sure why I'm getting this error. I'm following a video and trying to apply it to my existing post model but am still having trouble. Thanks
New Post Form:
<%= form_for :post, html: { multipart: true, class: "post-form" }, url: posts_path do |f| %>
<h4>Choose a Classroom</h4>
<div class="form-group">
<%= f.collection_select :classroom_id, current_user.classrooms, :id, :name %>
</div>
<h4>Write a post</h4>
<div class="form-group">
<%= f.text_area :content, class: "form-control" %>
</div>
<h4>Attach Files</h4>
<div class="form-group">
<%= f.file_field :image %>
</div>
<%= f.submit "Add Post", class: "btn btn-primary" %>
<% end %>
For a post reply
<li>
<%= post.content %> -
<%= form_for [post, Post.new] do |f| %>
<%= f.text_area :content, placeholder: "Add a Reply" %><br/>
<%= f.submit "Reply" %>
<% end %>
</li>

Answer 1

The error is in find_postable method, specifically in this line:
#postable = Classroom.find_by_id(params[:classroom_id]) if params[:classroom_id]
You are trying to fetch classroom_id from params (i.e. params[:classroom_id]) but that key (classroom_id) exists within post key in your parameters hash:
Parameters: {
"utf7"=>"✓",
"authenticity_token"=>"bunchofletters==",
"post"=>{
"classroom_id"=>"1",
"content"=>"hello"
},
"commit"=>"Add Post"
}
To fix this, change params[:classroom_id] to params[:post][:classroom_id] in find_postable method:
def find_postable
#postable = Post.find_by_id(params[:post_id]) if params[:post_id]
#postable = Classroom.find_by_id(params[:post][:classroom_id]) if params[:classroom_id]
end