Erro ao chamar send_email: *** ActiveRecord::RecordInvalid Exception: A validação falhou: User é obrigatório(a)

credit_card.rb

class Payment::PostProcessing::CreditCard
  def initialize(transaction)
    @transaction = transaction
  end

  def after_payment(order)
    @order = order
    register_amount_paid
    @order.pay!
    save_credit_card
    send_email
  end

  private
  def register_amount_paid
    @order.update_column('paid_centavos', @transaction.paid_amount)
  end

  def save_credit_card
    card = {}
    card['brand']        = @transaction.card.brand
    card['first_digits'] = @transaction.card.first_digits
    card['last_digits']  = @transaction.card.last_digits
    card['holder_name']  = @transaction.card.holder_name
    card['country']      = @transaction.card.country

    @order.card_info = card
    @order.save!
  end

  def send_email
    letter = Email::Letter.create!(
      context: 'order_completed',
      object:  @order,
      user_email: @order.shipping_email,
      email_template_id: Email::Template.find_by!(name: 'order_completed').id
    )
    begin
      letter.send_email
    rescue SparkPostRails::DeliveryException => e
      "We can't send you email because #{e.service_description}"
    end
  end
end

letter.rb

class Email::Letter < ApplicationRecord
  belongs_to :object, polymorphic: true, inverse_of: :email_letters
  belongs_to :user
  belongs_to :email_template, class_name: 'Email::Template'

  validates :user_email, presence: true, unless: :user_id?
  validates :context, presence: true, inclusion: { in: %w(admin_updated_store admin_deleted_store user_signed_up
user_created_store user_created_campaign product_was_sold order_completed order_canceled first_sale boleto_paid boleto_waiting
payment_request_user payment_request_admin payment_request_confirmed order_completed_vendor) }
  validates :status, presence: true, inclusion: { in: %w(init sending sent has_error) }

  validates :subject, presence: true, length: { minimum: 5, maximum: 60 }, unless: :email_template_id?
  validates :body, presence: true, length: { minimum: 30 }, unless: :email_template_id?

  include AASM

  aasm column: :status do
    state :init, initial: true
    state :sending
    state :sent
    state :has_error

    event :start_sending do
      transitions from: :init, to: :sending
    end

    event :sending_was_successful do
      transitions from: :sending, to: :sent
    end

    event :sending_was_unsuccessful do
      transitions from: :sending, to: :has_error
    end
  end

  def send_email
    self.start_sending!

    response = NotifyUserMailer.letter(self).deliver!

    if response['total_accepted_recipients'] >= 1

      self.update_attribute(:sparkpost_id, response['id'])
      self.sending_was_successful!
    else
      self.sending_was_unsuccessful!
    end

  end

  def replace_substitutions(text)
    Rails.application.routes.default_url_options[:host] = ENV['SITE_HOST']

    case context
      when 'admin_updated_store', 'admin_deleted_store', 'user_created_store'
        text.gsub!('%store.title%',         object.title)
        text.gsub!('%store.name%',          object.title)
        text.gsub!('%store.url%',           Rails.application.routes.url_helpers.store_url(object.slug))
        text.gsub!('%store.dashboard_url%', Rails.application.routes.url_helpers.edit_dashboard_store_url(uuid: object.uuid))

      when 'user_created_campaign'
        text.gsub!('%campaign.title%',         object.title)
        text.gsub!('%campaign.name%',          object.title)
        text.gsub!('%campaign.url%',           Rails.application.routes.url_helpers.campaign_url(id: object.slug))
        text.gsub!('%campaign.dashboard_url%', Rails.application.routes.url_helpers.edit_dashboard_campaign_url(uuid: object.uuid))

      when 'order_completed'
        text.gsub!('%order.list_of_products%', '<ul>' + object.order_products.map { |p| "<li>(#{p.quantity}x) #{p.product.campaign.title}</li>" }.join + '</ul>')
        text.gsub!('%order.subtotal%', ActionController::Base.helpers.humanized_money_with_symbol(object.common_price))
        text.gsub!('%order.shipping_price%', ActionController::Base.helpers.humanized_money_with_symbol(object.shipping_price))
        text.gsub!('%order.total_price%', ActionController::Base.helpers.humanized_money_with_symbol(object.total_price))

      when 'boleto_waiting'
        text.gsub!('%order.boleto_url%', object.boleto_url)
        text.gsub!('%order.list_of_products%', '<ul>' + object.order_products.map { |p| "<li>(#{p.quantity}x) #{p.product.campaign.title}</li>" }.join + '</ul>')
        text.gsub!('%order.subtotal%', ActionController::Base.helpers.humanized_money_with_symbol(object.common_price))
        text.gsub!('%order.shipping_price%', ActionController::Base.helpers.humanized_money_with_symbol(object.shipping_price))
        text.gsub!('%order.total_price%', ActionController::Base.helpers.humanized_money_with_symbol(object.total_price))

      when 'payment_request_user'
        text.gsub!('%request.amount%', ActionController::Base.helpers.humanized_money_with_symbol((object.amount_centavos)/100.0))

      when 'payment_request_admin'
        text.gsub!('%request.amount%', ActionController::Base.helpers.humanized_money_with_symbol(object.amount_centavos))

      when 'order_completed_vendor'
        order_products = object.order_products.joins(product: :campaign).where(campaigns: { user_id: user.id })
        text.gsub!('%order.list_of_products%', '<ul>' + order_products.map { |p| "<li>#{p.product.campaign.title}</li>" }.join + '</ul>')
        text.gsub!('%order.total_price%', ActionController::Base.helpers.humanized_money_with_symbol(order_products.sum(:price_centavos).to_f/100))

    end

    if context != 'order_completed' &&
        context != 'boleto_waiting' &&
        context != 'boleto_paid' &&
        context != 'payment_request_user' &&
        context != 'payment_request_admin' &&
        context != 'payment_request_confirmed'
      text.gsub!('%user.name%', user.name)
      text.gsub!('%user.full_name%', user.name)
      text.gsub!('%user.first_name%', user.first_name.to_s)
      text.gsub!('%user.last_name%',  user.last_name.to_s)
      text.gsub!('%user.email%',  user.email)
      text.gsub!('%user.full_email%',  user.full_email)
    end

    text
  end
end

# admin_updated_store     store     store.user
# admin_deleted_store     store     store.user
# user_signed_up          user      user
# user_created_store      store     store.user
# user_created_campaign   campaign  campaign.user
# product_was_sold        product   product.user
# order_completed         order     email
# order_canceled          order     email
# first_sale              product   product.user

O @order não tem user_id, e mesmo colocando um na mão no console, o erro persiste.

Este model do Email::Letter tem um belongs_to :user

validates :user_email, presence: true, unless: :user_id?

Tem essa validação e não vi campo user_email neste modo e não sei o motivo do user_id não está sendo atribuido

Exemplo (o erro acontece mesmo salvando um user_id):

letter = Email::Letter.create!(context: 'order_completed',object:Order.last,user_email: Order.last.shipping_email,email_template_id: Email::Template.find_by!(name: 'order_completed').id)
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" ORDER BY "orders"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Order Load (0.3ms)  SELECT  "orders".* FROM "orders" ORDER BY "orders"."id" DESC LIMIT $1  [["LIMIT", 1]]
  Email::Template Load (0.3ms)  SELECT  "email_templates".* FROM "email_templates" WHERE "email_templates"."name" = $1 ORDER BY "email_templates"."title" ASC LIMIT $2  [["name", "order_completed"], ["LIMIT", 1]]
   (0.1ms)  BEGIN
  Email::Template Load (0.2ms)  SELECT  "email_templates".* FROM "email_templates" WHERE "email_templates"."id" = $1 ORDER BY "email_templates"."title" ASC LIMIT $2  [["id", 6], ["LIMIT", 1]]
   (0.1ms)  ROLLBACK
ActiveRecord::RecordInvalid: A validação falhou: User é obrigatório(a)
from /Users/alexandremagno/.rvm/gems/ruby-2.3.1@usinademarcas/gems/activerecord-5.0.6/lib/active_record/validations.rb:78:in `raise_validation_error'

Mesmo problema aqui