Work around core library class mocks/stubs in ruby

Just a short post about mocking/stubbing your Ruby functions. Let's have a class like this:

class ConfigurableClass
  attr_reader :option
  def configure_from_file(file)
    json = JSON(IO.read(file))
    @option = json['my']['option']
  end
end

One way to test this would be to mock IO.read with Mocha

describe ConfigurableClass do
 it "can configure from json" do
   data =  '{ "my": { "option" : "hello" } }'
     IO.any_instance.expect(:read).with("x.json").returns(data)
     cc = ConfigurableClass.new
     cc.configure_from_file("x.json")
     cc.option.should eq "hello"
   end
end

This is tricky if your code does IO.read anywhere else which you obviously don't want to mock. However with some refactoring code is much cleaner.

class ConfigurableClass
  attr_reader :option
  def read_config_file(file)
    IO.read(file)
  end

  def configure_from_file(file)
    json = JSON(read_config_file)
    @option = json['my']['option']
  end
end

Then in the rspec it's easy to mock only function specific to ConfigurableClass.

describe ConfigurableClass do
  it "can configure from json" do
    data =  '{ "my": { "option" : "hello" } }'
    ConfigurableClass.any_instance.expect(read_config_file).with("x.json").returns(data)
    cc = ConfigurableClass.new
    cc.configure_from_file("x.json")
    cc.option.should eq "hello"
end