problem.rb

# config/routes.rb
Rails.application.routes.draw do
  resources :responders, except: [:new, :edit, :destroy], defaults: { format: :json }
  resources :emergencies, except: [:new, :edit, :destroy], defaults: { format: :json }

  match '*path', to: 'application#render_404', as: :render_404, via: :all
end

# app/controllers/emergency_controller.rb
class EmergenciesController < ApplicationController
  before_action :set_emergency, only: [:show]
  respond_to :json
  # GET /emergencies
  # GET /emergencies.json
  def index
  end

  # GET /emergencies/1
  # GET /emergencies/1.json
  def show
  end

  # POST /emergencies
  # POST /emergencies.json
  def create
  end

  # PATCH/PUT /emergencies/1
  # PATCH/PUT /emergencies/1.json
  def update
  end

  private
    def set_emergency
      @emergency = Emergency.find(params[:id])
    end
    # Never trust parameters from the scary internet, only allow the white list through.
    def emergency_params
      params.require(:emergency).permit(:code, :police_severity, :fire_severity, :medical_severity, :resolved_at)
    end
end

# app/controllers/application_controller.rb
class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception

  def render_404
    render json: { message: "page not found" }, status: 404
  end
end

1. Error:
   Emergencies404Test#test_404_on_GET_/emergencies/new:
   ActiveRecord::RecordNotFound: Couldn't find Emergency with 'id'=new