transfer.rb

class Transfer

	def initialize(payment)
		@payment = payment
	end

	def run
		ActiveRecord::Base.transaction do
			if @payment.origin.bank == @payment.destination.bank
				intra_bank
			else
				inter_bank
			end
			update_balances
		end
	end

	def intra_bank
		@payment.comision = 0
		@payment.status = "OK"
		@payment.save!
	end

	def inter_bank
		@payment.comision = 5
		@payment.status = "OK"
		@payment.save!
	end

	def update_balances
		origin = @payment.origin
		origin.balance = origin.balance - @payment.amount - @payment.comision
		origin.save!

		destination = @payment.destination
		destination.balance = destination.balance + @payment.amount
		destination.save!
	end

end

transfer_spec.rb

require "rails_helper"

describe Transfer do
  let(:user) { create(:user) }
  let(:bank) { FactoryBot.create(:bank) }
  let(:origin_account){ FactoryBot.create(:bank_account, bank: bank, balance: 15) }
  let(:destination_account) { FactoryBot.create(:bank_account, bank: bank, balance: 10) }
  let(:payment_intra_bank){ FactoryBot.create(:payment,
                                              amount: 1,
                                              origin: origin_account,
                                              destination: destination_account,
                                              kind: "transfer" )}

  let(:bank2) { FactoryBot.create(:bank) }
  let(:destination_account_different_bank){ FactoryBot.create(:bank_account, bank: bank2, balance: 10) }
  let(:payment_inter_bank){ FactoryBot.create(:payment,
                                              amount: 1,
                                              origin: origin_account,
                                              destination: destination_account_different_bank,
                                              kind: "transfer" )}

  it "transfer intra bank" do
    transfer = payment_intra_bank.payment_service
    result = transfer.new(payment_intra_bank).run!

    expect(result).to be_truthy
    expect(payment_intra_bank.comision).to eq(0)
    expect(payment_intra_bank.origin.balance).to eq(14)
    expect(payment_intra_bank.destination.balance).to eq(11)
  end

  it "transfer inter bank" do
    transfer = payment_inter_bank.payment_service
    result = transfer.new(payment_inter_bank).run!

    expect(result).to be_truthy
    expect(payment_inter_bank.comision).to eq(5)
    expect(payment_inter_bank.origin.balance).to eq(9)
    expect(payment_inter_bank.destination.balance).to eq(11)
  end

  it "transfer fails" do
    transfer = payment_inter_bank.payment_service
    origin = payment_inter_bank.origin
    origin.update_columns(balance: 0)
    
    result = transfer.new(payment_inter_bank).run!

    expect(result).to be_falsey
  end

end