Rails can't render JSON undefined method `new' for nil:NilClass - ruby-on-rails

Question

Background
I'm making a Rails application which has two namespaces, which are admin (for administrator purpose) and api for mobile & web client.
A weird thing happened yesterday. I made a new table facilities which consists of id, name, created_at, updated_at in my PostgreSQL database.
Problem
I tried to get all facilities using admin namespace http://localhost:3000/admin/facilities and it works well (return the HTML and the list of facilities).
Started GET "/admin/facilities" for ::1 at 2015-05-10 16:12:47 +0800
Processing by Admin::FacilitiesController#index as HTML
Facility Load (0.4ms) SELECT "facilities".* FROM "facilities"
Rendered admin/facilities/index.html.erb within layouts/application (2.5ms)
Completed 200 OK in 91ms (Views: 90.4ms | ActiveRecord: 0.4ms)
But when I call http://localhost:3000/api/v1/facilities from browser, which supposed to return JSON. I got an error
NoMethodError in Api::V1::FacilitiesController#index
undefined method `new' for nil:NilClass
Extracted source (around line #8):
6 def index
7 #facilities = Facility.all
8 render json: #facilities
9 end
10
11
The error from console
Started GET "/api/v1/facilities" for ::1 at 2015-05-10 16:38:26 +0800
Processing by Api::V1::FacilitiesController#index as HTML
User Load (0.6ms) SELECT "users".* FROM "users" WHERE "users"."id" IS NULL ORDER BY "users"."id" ASC LIMIT 1
Facility Load (0.5ms) SELECT "facilities".* FROM "facilities"
Completed 500 Internal Server Error in 6ms (ActiveRecord: 1.1ms)
NoMethodError (undefined method `new' for nil:NilClass):
app/controllers/api/v1/facilities_controller.rb:8:in `index'
Rendered /Users/abrahamks/.rvm/gems/ruby-2.2.1/gems/actionpack-4.2.1/lib/action_dispatch/middleware/templates/rescues/_source.erb (7.1ms)
Rendered /Users/abrahamks/.rvm/gems/ruby-2.2.1/gems/actionpack-4.2.1/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (2.7ms)
. . . .
. . . .
and so on
I check from rails c and call Facility.all and looks like the command line return correct values, but I don't understand why it can't render / return json.
Loading development environment (Rails 4.2.1)
2.2.1 :001 > Facility.all
Facility Load (0.5ms) SELECT "facilities".* FROM "facilities"
=> #<ActiveRecord::Relation [#<Facility id: 1, name: "Kitchen", created_at: "2015-05-09 10:54:00", updated_at: "2015-05-09 16:08:48">, #<Facility id: 2, name: "Washer", created_at: "2015-05-09 11:20:40", updated_at: "2015-05-09 16:09:32">, #<Facility id: 3, name: "Swimming Pool", created_at: "2015-05-09 11:22:19", updated_at: "2015-05-09 16:09:41">, #<Facility id: 4, name: "Internet", created_at: "2015-05-09 11:24:02", updated_at: "2015-05-09 16:12:31">, #<Facility id: 5, name: "Dryer", created_at: "2015-05-09 15:55:36", updated_at: "2015-05-09 16:12:54">]>
What's wrong with my code? Is there any configuration that I missed?
Thank you.
More Details if Needed
# GET /facilities/1
facilities has many-to-many relationship with properties, I generate a join table
class CreateJoinTablePropertiesFacilities < ActiveRecord::Migration
def change
create_join_table :properties, :facilities do |t|
t.index [:property_id, :facility_id]
t.index [:facility_id, :property_id]
end
end
end
Here is my facility.rb
class Facility < ActiveRecord::Base
has_and_belongs_to_many :property, join_table: :facilites_properties
end
Here is my routes.rb
Rails.application.routes.draw do
namespace :admin do
resources :facilities
end
namespace :api do
namespace :v1 do
resources :properties
resources :facilities, only: [:index, :show]
resources :users do
member do
post 'list', :to => "users#list_property"
end
end
Here is my app/controllers/api/v1/facilities_controller
class Api::V1::FacilitiesController < ApplicationController
before_action :set_facility, only: [:show, :edit, :update, :destroy]
# before_action :authenticate
# GET /facilities
# GET /facilities.json
def index
#facilities = Facility.all
render json: #facilities
end
# GET /facilities/1
# GET /facilities/1.json
def show
render json: #facility
end
private
# Use callbacks to share common setup or constraints between actions.
def set_facility
#facility = Facility.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def facility_params
params.require(:facility).permit(:name)
end
end
And this is app/controllers/admin/facilities_controller
class Admin::FacilitiesController < ApplicationController
before_action :set_facility, only: [:show, :edit, :update, :destroy]
# before_action :authenticate
# GET /facilities
# GET /facilities.json
def index
#facilities = Facility.all
end
# GET /facilities/1
# GET /facilities/1.json
def show
end
# GET /facilities/new
def new
#facility = Facility.new
end
# GET /facilities/1/edit
def edit
end
# POST /facilities
# POST /facilities.json
def create
#facility = Facility.new(facility_params)
respond_to do |format|
if #facility.save
format.html { redirect_to admin_facilities_path, notice: 'Facility was successfully created.' }
format.json { render :show, status: :created, location: #facility }
else
format.html { render :new }
format.json { render json: #facility.errors, status: :unprocessable_entity }
end
end
end
# PATCH/PUT /facilities/1
# PATCH/PUT /facilities/1.json
def update
respond_to do |format|
if #facility.update(facility_params)
format.html { redirect_to admin_facilities_path, notice: 'Facility was successfully updated.' }
format.json { render :show, status: :ok, location: #facility }
else
format.html { render :edit }
format.json { render json: #facility.errors, status: :unprocessable_entity }
end
end
end
# DELETE /facilities/1
# DELETE /facilities/1.json
def destroy
#facility.destroy
respond_to do |format|
format.html { redirect_to facilities_url, notice: 'Facility was successfully destroyed.' }
format.json { head :no_content }
end
end
private
# Use callbacks to share common setup or constraints between actions.
def set_facility
#facility = Facility.find(params[:id])
end
# Never trust parameters from the scary internet, only allow the white list through.
def facility_params
params.require(:facility).permit(:name)
end
end

Answer 1

The stack trace is showing the request was processed as HTML.
Processing by Admin::FacilitiesController#index as HTML
As you are trying to render JSON, you can either specify the controller to respond to json
module Api
module V1
class FacilitiesController < ApplicationController
respond_to :json
end
end
end
Or you can specify json as a default response within your routes.rb file
namespace :api, defaults: { format: 'json' } do
# your api json routes...
end
Hope either one of these help.

Answer 2

I just realised I forgot to generate the serializer. Since I use gem Active Model Serializers, this is what I did to generate the serializer.
rails g serializer facility