Exploration Through Example

Example-driven development, Agile testing, context-driven testing, Agile programming, Ruby, and other things of interest to Brian Marick

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Mon, 12 Apr 2004

Ruby automocks

Michael Feathers asks for a way for tests to substitute their own mock classes for any class used, however indirectly, by the class under test. That would be simpler than refactoring to pass mocks around in the code under test, though perhaps, in the long run, less desirable. As Michael says, "I'm not convinced that is great thing. Abused it would just let code get uglier without bound. But, used well, it could really help people out of some jams."

I undid a bad mood this morning by hacking up something like that for Ruby. Here's an example:

require 'test/unit'
require 'automock'

module DBI
  # This is a utility used by the class under test.
  class BigHairyDatabaseConnection
    def name
      "true database"
    end
  end
end

module SeparateAppPackage
  # This class will use the big hairy database connection.
  # How to mock that out without changing this code?
  class ClassUnderTest
    def initialize
      @database = DBI::BigHairyDatabaseConnection.new
    end

    def which_database
      @database.name
    end
  end
end

class TestSeparatePackage < Test::Unit::TestCase
  include SeparateAppPackage

  # Mock out the big hairy database connection.			    
  mock DBI::BigHairyDatabaseConnection

  # Here's what it gets mocked out with.
  class MockBigHairyDatabaseConnection
    def name
      "mock database"
    end
  end
  
  def test_mock
    # Notice that the ClassUnderTest magically uses the mock,
    # not the class it was coded to use.
    assert_equal("mock database",
                 ClassUnderTest.new.which_database)
  end
end

This is just a spike, though I don't offhand see any impediments to a solid implementation. I can finish it up or hand it off if anyone would actually use it.

## Posted at 18:59 in category /ruby [permalink] [top]