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469 lines
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469 lines
15 KiB
Text
h2. The Basics of Creating Rails Plugins
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A Rails plugin is either an extension or a modification of the core framework. Plugins provide:
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* a way for developers to share bleeding-edge ideas without hurting the stable code base
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* a segmented architecture so that units of code can be fixed or updated on their own release schedule
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* an outlet for the core developers so that they don’t have to include every cool new feature under the sun
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After reading this guide you should be familiar with:
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* Creating a plugin from scratch
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* Writing and running tests for the plugin
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This guide describes how to build a test-driven plugin that will:
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* Extend core ruby classes like Hash and String
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* Add methods to ActiveRecord::Base in the tradition of the 'acts_as' plugins
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* Give you information about where to put generators in your plugin.
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For the purpose of this guide pretend for a moment that you are an avid bird watcher.
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Your favorite bird is the Yaffle, and you want to create a plugin that allows other developers to share in the Yaffle
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goodness.
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endprologue.
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h3. Setup
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h4. Generating the Plugin Skeleton
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Rails currently ships with a generator to generate a plugin within a Rails application. Help text is available that will explain
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how this generator works.
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<shell>
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$ rails generate plugin --help
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</shell>
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This generator places the plugin into the vendor/plugins directory.
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Vendored plugins are useful for quickly prototyping your plugin but current thinking in the Rails community is shifting towards
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packaging plugins as gems, especially with the inclusion of Bundler as the Rails dependency manager.
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Packaging a plugin as a gem may be overkill for any plugins that will not be shared across projects but doing so from the start makes it easier to share the plugin going forward without adding too much additional overhead during development.
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Rails 3.1 will ship with a plugin generator that will default to setting up a plugin
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as a gem. This tutorial will begin to bridge that gap by demonstrating how to create a gem based plugin using the
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"Enginex gem":http://www.github.com/josevalim/enginex.
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<shell>
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$ gem install enginex
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$ enginex --help
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$ enginex yaffle
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</shell>
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This command will create a new directory named "yaffle" within the current directory.
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h3. Testing your newly generated plugin
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You can navigate to the directory that contains the plugin, run the +bundle install+ command
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and run the one generated test using the +rake+ command.
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You should see:
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<shell>
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2 tests, 2 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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This will tell you that everything got generated properly and you are ready to start adding functionality.
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h3. Extending Core Classes
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This section will explain how to add a method to String that will be available anywhere in your rails application.
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In this example you will add a method to String named +to_squawk+. To begin, create a new test file with a few assertions:
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<ruby>
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# yaffle/test/core_ext_test.rb
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require 'test_helper'
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class CoreExtTest < Test::Unit::TestCase
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def test_to_squawk_prepends_the_word_squawk
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assert_equal "squawk! Hello World", "Hello World".to_squawk
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end
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end
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</ruby>
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Run +rake+ to run the test. This test should fail because we haven't implemented the +to_squak+ method:
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<shell>
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1) Error:
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test_to_squawk_prepends_the_word_squawk(CoreExtTest):
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NoMethodError: undefined method `to_squawk' for "Hello World":String
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test/core_ext_test.rb:5:in `test_to_squawk_prepends_the_word_squawk'
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</shell>
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Great - now you are ready to start development.
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Then in +lib/yaffle.rb+ require +lib/core_ext+:
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<ruby>
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# yaffle/lib/yaffle.rb
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require "yaffle/core_ext"
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module Yaffle
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end
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</ruby>
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Finally, create the +core_ext.rb+ file and add the +to_squawk+ method:
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<ruby>
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# yaffle/lib/yaffle/core_ext.rb
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String.class_eval do
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def to_squawk
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"squawk! #{self}".strip
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end
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end
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</ruby>
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To test that your method does what it says it does, run the unit tests with +rake+ from your plugin directory.
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<shell>
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3 tests, 3 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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To see this in action, change to the test/dummy directory, fire up a console and start squawking:
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<shell>
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$ rails console
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>> "Hello World".to_squawk
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=> "squawk! Hello World"
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</shell>
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h3. Add an "acts_as" Method to Active Record
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A common pattern in plugins is to add a method called 'acts_as_something' to models. In this case, you
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want to write a method called 'acts_as_yaffle' that adds a 'squawk' method to your Active Record models.
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To begin, set up your files so that you have:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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end
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</ruby>
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<ruby>
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# yaffle/lib/yaffle.rb
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require "yaffle/core_ext"
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require 'yaffle/acts_as_yaffle'
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module Yaffle
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end
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</ruby>
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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# your code will go here
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end
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end
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</ruby>
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h4. Add a Class Method
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This plugin will expect that you've added a method to your model named 'last_squawk'. However, the
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plugin users might have already defined a method on their model named 'last_squawk' that they use
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for something else. This plugin will allow the name to be changed by adding a class method called 'yaffle_text_field'.
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To start out, write a failing test that shows the behavior you'd like:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
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assert_equal "last_squawk", Hickwall.yaffle_text_field
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end
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def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
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assert_equal "last_tweet", Wickwall.yaffle_text_field
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end
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end
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</ruby>
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When you run +rake+, you should see the following:
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<shell>
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1) Error:
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test_a_hickwalls_yaffle_text_field_should_be_last_squawk(ActsAsYaffleTest):
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NameError: uninitialized constant ActsAsYaffleTest::Hickwall
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test/acts_as_yaffle_test.rb:6:in `test_a_hickwalls_yaffle_text_field_should_be_last_squawk'
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2) Error:
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test_a_wickwalls_yaffle_text_field_should_be_last_tweet(ActsAsYaffleTest):
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NameError: uninitialized constant ActsAsYaffleTest::Wickwall
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test/acts_as_yaffle_test.rb:10:in `test_a_wickwalls_yaffle_text_field_should_be_last_tweet'
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5 tests, 3 assertions, 0 failures, 2 errors, 0 skips
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</shell>
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This tells us that we don't have the necessary models (Hickwall and Wickwall) that we are trying to test.
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We can easily generate these models in our "dummy" Rails application by running the following commands from the
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test/dummy directory:
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<shell>
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$ cd test/dummy
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$ rails generate model Hickwall last_squak:string
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$ rails generate model Wickwall last_squak:string last_tweet:string
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</shell>
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Now you can create the necessary database tables in your testing database by navigating to your dummy app
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and migrating the database. First
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<shell>
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$ cd test/dummy
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$ rake db:migrate
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$ rake db:test:prepare
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</shell>
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While you are here, change the Hickwall and Wickwall models so that they know that they are supposed to act
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like yaffles.
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<ruby>
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# test/dummy/app/models/hickwall.rb
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class Hickwall < ActiveRecord::Base
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acts_as_yaffle
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end
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# test/dummy/app/models/wickwall.rb
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class Wickwall < ActiveRecord::Base
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acts_as_yaffle :yaffle_text_field => :last_tweet
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end
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</ruby>
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We will also add code to define the acts_as_yaffle method.
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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# your code will go here
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end
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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You can then return to the root directory (+cd ../..+) of your plugin and rerun the tests using +rake+.
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<shell>
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1) Error:
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test_a_hickwalls_yaffle_text_field_should_be_last_squawk(ActsAsYaffleTest):
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NoMethodError: undefined method `yaffle_text_field' for #<Class:0x000001016661b8>
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/Users/xxx/.rvm/gems/ruby-1.9.2-p136@xxx/gems/activerecord-3.0.3/lib/active_record/base.rb:1008:in `method_missing'
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test/acts_as_yaffle_test.rb:5:in `test_a_hickwalls_yaffle_text_field_should_be_last_squawk'
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2) Error:
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test_a_wickwalls_yaffle_text_field_should_be_last_tweet(ActsAsYaffleTest):
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NoMethodError: undefined method `yaffle_text_field' for #<Class:0x00000101653748>
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Users/xxx/.rvm/gems/ruby-1.9.2-p136@xxx/gems/activerecord-3.0.3/lib/active_record/base.rb:1008:in `method_missing'
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test/acts_as_yaffle_test.rb:9:in `test_a_wickwalls_yaffle_text_field_should_be_last_tweet'
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5 tests, 3 assertions, 0 failures, 2 errors, 0 skips
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</shell>
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Getting closer...now we will implement the code of the acts_as_yaffle method to make the tests pass.
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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cattr_accessor :yaffle_text_field
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self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
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end
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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When you run +rake+ you should see the tests all pass:
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<shell>
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5 tests, 5 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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h4. Add an Instance Method
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This plugin will add a method named 'squawk' to any Active Record objects that call 'acts_as_yaffle'. The 'squawk'
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method will simply set the value of one of the fields in the database.
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To start out, write a failing test that shows the behavior you'd like:
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<ruby>
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# yaffle/test/acts_as_yaffle_test.rb
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require 'test_helper'
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class ActsAsYaffleTest < Test::Unit::TestCase
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def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
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assert_equal "last_squawk", Hickwall.yaffle_text_field
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end
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def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
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assert_equal "last_tweet", Wickwall.yaffle_text_field
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end
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def test_hickwalls_squawk_should_populate_last_squawk
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hickwall = Hickwall.new
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hickwall.squawk("Hello World")
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assert_equal "squawk! Hello World", hickwall.last_squawk
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end
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def test_wickwalls_squawk_should_populate_last_tweeted_at
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wickwall = Wickwall.new
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wickwall.squawk("Hello World")
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assert_equal "squawk! Hello World", wickwall.last_tweet
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end
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end
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</ruby>
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Run the test to make sure the last two tests fail the an error that contains "NoMethodError: undefined method `squawk'",
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then update 'acts_as_yaffle.rb' to look like this:
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<ruby>
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# yaffle/lib/yaffle/acts_as_yaffle.rb
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module Yaffle
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module ActsAsYaffle
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extend ActiveSupport::Concern
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included do
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end
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module ClassMethods
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def acts_as_yaffle(options = {})
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cattr_accessor :yaffle_text_field
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self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
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end
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end
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def squawk(string)
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write_attribute(self.class.yaffle_text_field, string.to_squawk)
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end
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end
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end
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ActiveRecord::Base.send :include, Yaffle::ActsAsYaffle
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</ruby>
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Run +rake+ one final time and you should see:
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<shell>
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7 tests, 7 assertions, 0 failures, 0 errors, 0 skips
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</shell>
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NOTE: The use of +write_attribute+ to write to the field in model is just one example of how a plugin can
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interact with the model, and will not always be the right method to use. For example, you could also
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use +send("#{self.class.yaffle_text_field}=", string.to_squawk)+.
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h3. Generators
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Generators can be included in your gem simply by creating them in a lib/generators directory of your plugin. More information about
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the creation of generators can be found in the "Generators Guide":generators.html
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h3. Publishing your Gem
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Gem plugins in progress can be easily be shared from any Git repository. To share the Yaffle gem with others, simply
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commit the code to a Git repository (like Github) and add a line to the Gemfile of the any application:
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<ruby>
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gem 'yaffle', :git => 'git://github.com/yaffle_watcher/yaffle.git'
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</ruby>
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After running +bundle install+, your gem functionality will be available to the application.
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When the gem is ready to be shared as a formal release, it can be published to "RubyGems":http://www.rubygems.org.
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For more information about publishing gems to RubyGems, see: "http://blog.thepete.net/2010/11/creating-and-publishing-your-first-ruby.html":http://blog.thepete.net/2010/11/creating-and-publishing-your-first-ruby.html
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h3. Non-Gem Plugins
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Non-gem plugins are useful for functionality that won't be shared with another project. Keeping your custom functionality in the
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vendor/plugins directory un-clutters the rest of the application.
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Move the directory that you created for the gem based plugin into the vendor/plugins directory of a generated Rails application, create a vendor/plugins/yaffle/init.rb file that contains "require 'yaffle'" and everything will still work.
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<ruby>
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# yaffle/init.rb
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require 'yaffle'
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</ruby>
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You can test this by changing to the Rails application that you added the plugin to and starting a rails console. Once in the
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console we can check to see if the String has an instance method of to_squawk.
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<shell>
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$ cd my_app
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$ rails console
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$ String.instance_methods.sort
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</shell>
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You can also remove the .gemspec, Gemfile and Gemfile.lock files as they will no longer be needed.
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h3. RDoc Documentation
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Once your plugin is stable and you are ready to deploy do everyone else a favor and document it! Luckily, writing documentation for your plugin is easy.
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The first step is to update the README file with detailed information about how to use your plugin. A few key things to include are:
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* Your name
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* How to install
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* How to add the functionality to the app (several examples of common use cases)
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* Warning, gotchas or tips that might help save users time
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Once your README is solid, go through and add rdoc comments to all of the methods that developers will use. It's also customary to add '#:nodoc:' comments to those parts of the code that are not part of the public api.
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Once your comments are good to go, navigate to your plugin directory and run:
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<shell>
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$ rake rdoc
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</shell>
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h4. References
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* "Developing a RubyGem using Bundler":https://github.com/radar/guides/blob/master/gem-development.md
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* "Using Gemspecs As Intended":http://yehudakatz.com/2010/04/02/using-gemspecs-as-intended/
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* "Gemspec Reference":http://docs.rubygems.org/read/chapter/20
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* "GemPlugins":http://www.mbleigh.com/2008/06/11/gemplugins-a-brief-introduction-to-the-future-of-rails-plugins
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* "Keeping init.rb thin":http://daddy.platte.name/2007/05/rails-plugins-keep-initrb-thin.html
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h3. Changelog
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* March 10, 2011: Minor formatting tweaks.
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* February 13, 2011: Get guide in synch with Rails 3.0.3. Remove information not compatible with Rails 3. Send reader elsewhere
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for information that is covered elsewhere.
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* April 4, 2010: Fixed document to validate XHTML 1.0 Strict. "Jaime Iniesta":http://jaimeiniesta.com
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* November 17, 2008: Major revision by Jeff Dean
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