API with Rails

application.rb

config.middleware.insert_before 0, "Rack::Cors", :debug => true, :logger => (-> { Rails.logger }) do
  allow do
    origins '*'

    resource '*',
    headers: :any,
    expose: 'X-Pagination',
    methods: [:get, :post, :delete, :put, :patch, :options, :head],
    max_age: 0
  end
end

Jbuilder.key_format camelize: :lower

base_controller.rb

class BaseController < ApplicationController
  rescue_from ActiveRecord::RecordNotFound, with: :record_not_found
  rescue_from ActionController::BadRequest, with: :bad_request

  skip_before_action :verify_authenticity_token

  before_action do
    params.deep_transform_keys!(&:underscore)
  end

  private

  def bad_request(error)
    render json: { error: error.message }, status: :bad_request
  end

  def record_not_found(error)
    render json: { error: error.message }, status: :not_found
  end
end

Gemfile

gem 'jbuilder'
gem 'rack-cors'