[RUBY] If you want to mock a method in RSpec, you should use the allow method for mock and the singleton method.

Overview

In rspec, you often want to mock a method to make it easier to test.

In the example below, the set_complex_config method affects the ʻadmin?` method.

The set_complex_config method already has its own test code and we only want the response.

#Code example is appropriate
class User
  def admin?
    res = set_complex_config
    case res
    when 'hoge'
      'hoge'
    when 'fuga'
      'fuga'
    end
  end

  def set_complex_config
    #Complex processing
  end
end

I can think of two ways to mock in such a case, so I thought about which one was better.

How to mock

① Define a singleton method and mock

How to overwrite the set_complex_config method of User class with singleton method as shown below.

let(:hoge_let) { 'hoge' }
it do
  user = create(:user)
  hoge_var = 'hoge'
  user.define_singleton_method(:set_complex_config) { hoge_var }
  user.set_complex_config =>Move

  user.define_singleton_method(:set_complex_config) { hoge_let }
  user.set_complex_config => wrong number of arguments (given 0, expected 2..3)
end

At first glance, it looks cool, but in this case, inside the example or before block, the block of the define_singleton_method method can't read the let variable.

② Mock with allow method

The allow method works fine.

let(:hoge_let) { 'hoge' }
it do
  user = create(:user)
  allow(user).to receive(:attributes).and_return(hoge_let)
  user.set_complex_config =>Move
end

Conclusion

It seems better to use ʻallow (). To receive (). And_return ()` obediently.

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