Rails - Model not saving to db - ruby-on-rails

Question

I'm completing this airbnb clone course (https://code4startup.com/projects/build-airbnb-with-ruby-on-rails-level-1) but have diverted a bit in order to complete my own project; a marketplace for education camps. Therefore I've added an additional model 'Courses'. It now has User>Listing>Course. This Courses model is working in rails console but not saving to my database when I'm running the server. Any suggestions would be appreciated...
Error Message
ActiveRecord::RecordInvalid in CoursesController#create
Validation failed: Listing must exist
Models
class User < ApplicationRecord
has_many :listings
has_many :courses, :through => :listings
end
class Listing < ApplicationRecord
belongs_to :user
has_many :courses
validates :listing_type, presence: true
validates :course_type, presence: true
validates :accommodate, presence: true
end
class Course < ApplicationRecord
belongs_to :listing
validates :curriculum_type, presence: true
validates :course_places, presence: true
end
Course Controller
class CoursesController < ApplicationController
before_action :set_course, except: [:index, :new, :create]
before_action :authenticate_user!, except: [:show]
def index
#courses = current_user.courses
end
def new
#course = current_user.courses.build
end
def create
#course = current_user.courses.build(course_params)
if #course.save!
redirect_to course_listing_path(#course), notice: "Saved..."
else
render :new, notice: "Something went wrong..."
end
end
def show
end
def listing
end
def pricing
end
def description
end
def photo_upload
end
def amenities
end
def location
end
def update
if #course.update(course_params)
flash[:notice] = "Saved..."
else
flash[:notice] = "Something went wrong..."
end
redirect_back(fallback_location: request.referer)
end
private
def set_course
#course = Course.find(params[:id])
end
def course_params
params.require(:course).permit(:name, :curriculum_type, :summary, :address, :course_places, :start_date, :finish_date, :price)
end
end
Routes
Rails.application.routes.draw do
root 'pages#home'
devise_for :users,
path: '',
path_names: {sign_in: 'login', sign_out: 'logout', edit: 'profile', sign_up: 'registration'},
controllers: { omniauth_callbacks: 'omniauth_callbacks', registrations: 'registrations' }
# For details on the DSL available within this file, see http://guides.rubyonrails.org/routing.html
resources :users, only: [:show]
resources :listings, except: [:edit] do
member do
get 'listing'
get 'pricing'
get 'description'
get 'photo_upload'
get 'amenities'
get 'location'
end
end
resources :courses, except: [:edit] do
member do
get 'listing'
get 'pricing'
get 'description'
get 'photo_upload'
get 'amenities'
get 'location'
end
end
end

Answer 1

below you can read my comments with the # sign
You are trying to save an object Course that has belongs_to listings so it is expected that it has as course.listing_id the id of an existing Listing
def create
#course = current_user.courses.build(course_params)
# You need to set #course.listing_id to an existing Listing
# You need to find that listing and save it in a variable.
# I am not getting into your logic, because your code is confused and need many adjustments
listing = Listing.find() # include hear your logic to find an existing listing from the db
#course.listing_id = listing.id
if #course.save!
redirect_to course_listing_path(#course), notice: "Saved..."
else
render :new, notice: "Something went wrong..."
end
end