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Professional Ruby on Rails™ by Noel Rappin

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15.4. Writing the Plugin

If your plugin doesn't change ActiveRecord::Base or affect Rails data structures in any way, then test-first development for your plugin is quite similar to test-first development of any code.

15.4.1. ActiveRecord Test Setup

If you are trying to modify ActiveRecord::Base in your plugin with an acts_as_whatever method, then you have some additional setup work to do. Ideally, you'd like to be able to do something like this:

require 'test/unit'
require "#{File.dirname(__FILE__)}/../../../../test/test_helper"

class ReviewableThing < ActiveRecord::Base
  acts_as_reviewable
end

class ActsAsReviewableTest < Test::Unit::TestCase

  def setup
    @rt = ReviewableThing
  end

  def test_this_plugin
    assert_responds_to @rt, :reviews
  end
end

Unfortunately, if you try that test exactly as written here, it will fail in the depths of ActiveRecord. ActiveRecord::Base assumes that it will be able to query a database to get information about its attributes. Because the ReviewableThing class doesn't have a database table associated with it, ActiveRecord fails to load the class.

This is another manifestation of the larger issue about plugins and testing. The plugin isn't really part of the project that uses it, so it can't depend on that project for test fixtures and setup. Broadly speaking, there are two options for working around this issue. You can either do a lot of setup work to give the plugin its own database setup, which introduces a dependency on a specific database in the developers ...

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