Rails MiniTest stub module method for the whole test

Question

I want to test my service where I must to replace one module method by integer 50 for whole test. To do so I'm using Stub:

# tested service

module CashTransactions
  class DistributeFunds
    def initialize(users:, amount:)
      @users = users
      @amount = amount
    end

    def call
      users.each do |user|
        funds = CashTransactionSettings.calculate_amount_distribution(user, amount)
        CashTransaction.create!(user: user, funds: funds)
      end
    end
  end
end

Based on this source I've prepared following test:

test 'create CashTransaction records' do
  user1_funds = CashTransactionSettings
  user2_funds = CashTransactionSettings

  refute user1_funds.calculate_amount_distribution
  refute user2_funds.calculate_amount_distribution

  CashTransactionSettings.stub :calculate_amount_distribution, 50 do
    assert user1_funds.calculate_amount_distribution(user1, 100), 50
    assert user2_funds.calculate_amount_distribution(user2, 100), 50

    assert_equal 1, CashTransaction.where(user: user1, amount: 50).size
    assert_equal 1, CashTransaction.where(user: user2, amount: 50).size
  end
end

But instead of replacing CashTransactionSettings.calculate_amount_distribution by 50 it calls it, where am I wrong? or maybe it's not possible in MiniTest?

Answer 1

You can use any_instance method and for that test block you will get the expected value as output of that method.

CashTransactionSettings.any_instance.stubs(:calculate_amount_distribution).returns(50)

And test case will be updated as below

test 'create CashTransaction records' do
  user1_funds = CashTransactionSettings
  user2_funds = CashTransactionSettings

  refute user1_funds.calculate_amount_distribution
  refute user2_funds.calculate_amount_distribution

  CashTransactionSettings.any_instance.stubs(:calculate_amount_distribution).returns(50)

  assert user1_funds.calculate_amount_distribution(user1, 100), 50
  assert user2_funds.calculate_amount_distribution(user2, 100), 50

  assert_equal 1, CashTransaction.where(user: user1, amount: 50).size
  assert_equal 1, CashTransaction.where(user: user2, amount: 50).size
end