after_create :create_braintree_customer

def create_braintree_customer
  Braintree.api_key = "bk-54cfa45"
  self.id = Braintree::Customer.create.id
  self.save
end

$ cat ~/project/app/models/user.rb
class User < ApplicationRecord
  # Include default devise modules. Others available are:
  # :confirmable, :lockable, :timeoutable and :omniauthable
  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :trackable, :validatable

  <<<<<<<< my code block should come here >>>>>>>>>>>>

end