contract.rb

class Booking::Contract < Dry::Validation::Contract
  params do
    required(:seansId).filled(:integer)
    required(:email).value(:string)
    required(:seatIds).value(:array).each(:integer).filled
  end

  rule(:email) do
    unless /\A[\w+\-.]+@[a-z\d\-]+(\.[a-z\d\-]+)*\.[a-z]+\z/i.match?(value)
      key.failure('has invalid format')
    end
  end
end

controller.rb

class BookingsController < ApplicationController
  skip_before_action :verify_authenticity_token

  def create
    result = Booking::Create.new.call(params)

    if result.success?
      render json: result.value!, status: 201
    else
      render json: result.failure[:errors], status: result.failure[:http_code]
    end
  end
end

dry.rb

require 'dry/monads'
require 'dry/monads/do'

class Booking::Create
  include Dry::Monads[:result]
  include Dry::Monads::Do.for(:call)

  def call(params)
    return Failure(errors: 'not authorized', http_code: 401) unless authorized?

    params = yield validate(params.to_unsafe_h)
    params = yield check_seats(params)
    bookings = yield create_booking(params.to_h)

    Success(bookings)
  end

  private

  def authorized?
    RestClient.get
  end

  def validate(params)
    result = Booking::Contract.new.call(params)

    if result.success?
      Success(result)
    else
      Failure(errors: result.errors.to_h, http_code: 422)
    end
  end

  def create_booking(params)
    bookings = ActiveRecord::Base.transaction do
      params[:seatIds].each do |seat_id|
        Booking.new(seat_id: seat_id, showing_id: params[:seansId], user: user(params[:email])).save!
      end
    end

    Success(bookings)
  rescue ActiveRecord::RecordNotUnique
    Failure(errors: { seat_busy: params[:seatIds] })
  end

  def check_seats(params)
    bookings = Booking.where(seat_id: params[:seatIds], showing_id: params[:seansId])

    if bookings.present?
      Failure(errors: { seat_busy: bookings.pluck(:seat_id) }, http_code: 409)
    else
      Success(params)
    end
  end

  def user(email)
    User.find_by(email: email) || User.create(email: email)
  end
end