== Add an `acts_as_yaffle` method to Active Record ==

A common pattern in plugins is to add a method called 'acts_as_something' to models.  In this case, you want to write a method called 'acts_as_yaffle' that adds a 'squawk' method to your models.

To begin, set up your files so that you have:

*vendor/plugins/yaffle/test/acts_as_yaffle_test.rb*

[source, ruby]
------------------------------------------------------
require File.dirname(__FILE__) + '/test_helper.rb'

class ActsAsYaffleTest < Test::Unit::TestCase
end
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*vendor/plugins/yaffle/lib/yaffle.rb*

[source, ruby]
------------------------------------------------------
require 'yaffle/acts_as_yaffle'
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*vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb*

[source, ruby]
------------------------------------------------------
module Yaffle
  # your code will go here
end
------------------------------------------------------

Note that after requiring 'acts_as_yaffle' you also have to include it into ActiveRecord::Base so that your plugin methods will be available to the rails models.

One of the most common plugin patterns for 'acts_as_yaffle' plugins is to structure your file like so:

*vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb*

[source, ruby]
------------------------------------------------------
module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    # any method placed here will apply to classes, like Hickwall
    def acts_as_something
      send :include, InstanceMethods
    end
  end

  module InstanceMethods
    # any method placed here will apply to instaces, like @hickwall
  end
end
------------------------------------------------------

With structure you can easily separate the methods that will be used for the class (like `Hickwall.some_method`) and the instance (like `@hickwell.some_method`).

=== Add a class method ===

This plugin will expect that you've added a method to your model named 'last_squawk'.  However, the plugin users might have already defined a method on their model named 'last_squawk' that they use for something else.  This plugin will allow the name to be changed by adding a class method called 'yaffle_text_field'.

To start out, write a failing test that shows the behavior you'd like:

*vendor/plugins/yaffle/test/acts_as_yaffle_test.rb*

[source, ruby]
------------------------------------------------------
require File.dirname(__FILE__) + '/test_helper.rb'

class Hickwall < ActiveRecord::Base
  acts_as_yaffle
end

class Wickwall < ActiveRecord::Base
  acts_as_yaffle :yaffle_text_field => :last_tweet
end

class ActsAsYaffleTest < Test::Unit::TestCase
  load_schema

  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end

  def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end
end
------------------------------------------------------

To make these tests pass, you could modify your `acts_as_yaffle` file like so:

*vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb*

[source, ruby]
------------------------------------------------------
module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    def acts_as_yaffle(options = {})
      cattr_accessor :yaffle_text_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
    end
  end
end

ActiveRecord::Base.send :include, Yaffle
------------------------------------------------------

=== Add an instance method ===

This plugin will add a method named 'squawk' to any Active Record objects that call 'acts_as_yaffle'.  The 'squawk' method will simply set the value of one of the fields in the database.

To start out, write a failing test that shows the behavior you'd like:

*vendor/plugins/yaffle/test/acts_as_yaffle_test.rb*

[source, ruby]
------------------------------------------------------
require File.dirname(__FILE__) + '/test_helper.rb'

class Hickwall < ActiveRecord::Base
  acts_as_yaffle
end

class Wickwall < ActiveRecord::Base
  acts_as_yaffle :yaffle_text_field => :last_tweet
end

class ActsAsYaffleTest < Test::Unit::TestCase
  load_schema

  def test_a_hickwalls_yaffle_text_field_should_be_last_squawk
    assert_equal "last_squawk", Hickwall.yaffle_text_field
  end

  def test_a_wickwalls_yaffle_text_field_should_be_last_tweet
    assert_equal "last_tweet", Wickwall.yaffle_text_field
  end
  
  def test_hickwalls_squawk_should_populate_last_squawk
    hickwall = Hickwall.new
    hickwall.squawk("Hello World")
    assert_equal "squawk! Hello World", hickwall.last_squawk
  end  
  
  def test_wickwalls_squawk_should_populate_last_tweeted_at
    wickwall = Wickwall.new
    wickwall.squawk("Hello World")
    assert_equal "squawk! Hello World", wickwall.last_tweet
  end  
end
------------------------------------------------------

Run this test to make sure the last two tests fail, then update 'acts_as_yaffle.rb' to look like this:

*vendor/plugins/yaffle/lib/yaffle/acts_as_yaffle.rb*

[source, ruby]
------------------------------------------------------
module Yaffle
  def self.included(base)
    base.send :extend, ClassMethods
  end

  module ClassMethods
    def acts_as_yaffle(options = {})
      cattr_accessor :yaffle_text_field
      self.yaffle_text_field = (options[:yaffle_text_field] || :last_squawk).to_s
      send :include, InstanceMethods
    end
  end

  module InstanceMethods
    def squawk(string)
      write_attribute(self.class.yaffle_text_field, string.to_squawk)
    end
  end
end

ActiveRecord::Base.send :include, Yaffle
------------------------------------------------------

.Editor's note:
NOTE: The use of `write_attribute` to write to the field in model is just one example of how a plugin can interact with the model, and will not always be the right method to use.  For example, you could also use `send("#{self.class.yaffle_text_field}=", string.to_squawk)`.