Extending Rails with Plugins (2007)

(QCon London, 2007)

"One of the most important and useful aspects of the Rails framework is it's ability to be extended using plugins. Thanks to the flexibility and power of Ruby, almost any aspect of Rails can be altered to suit the needs of a particular application.

"This presentation will help give developers the boost that's often required to get up to speed developing plugins. We'll cover the hooks that Rails' plugin mechanism makes available, and how to put them to best use in practice with practical examples.

"Once we've covered the groundwork, we'll start to look at more advanced programming techniques for sharing code (and other files) between Rails applications. With a few key programming techniques under our belt, we can use plugins to alter and enhance the Rails framework itself."

Transcript

1. Plugins
   James Adam
   Rails
   

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2. $ script/plugin install
   

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10. lib
    

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11. init.rb
    

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12. tasks install.rb
    uninstall.rb test
    

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13. Plugins
    developing
    

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14. Rails plugins
    can add
    anything
    

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15. Rails plugins
    can change
    anything
    

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16. Rails plugins
    can do
    anything
    

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17. Methods
    Adding
    with Ruby, and Rails
    

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18. Modules
    module MineNotYours
    def copyright
    "(c) me"
    end
    end
    

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19. Adding methods to instances...
    class SomeClass
    include MineNotYours
    end
    c = SomeClass.new
    c.copyright # => "(c) me"
    

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20. ... and so to Models
    class SomeModel < AR::Base
    include MineNotYours
    end
    m = SomeModel.find(:first)
    m.copyright # => "(c) me"
    

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21. Adding methods to all models
    # Re-opening the target class
    # directly
    class ActiveRecord::Base
    include MineNotYours
    end
    

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22. Adding methods to all models
    # Send the ‘include’ message to
    # our target class
    AR::Base.send(:include, MineNotYours)
    

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24. Behaviour
    Adding
    as a part of Class Definitions
    

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25. Plain Ol’ Ruby Objects
    class ConferenceDelegate
    attr_accessor :name
    end
    c = ConferenceDelegate.new
    c.name = "Joe Blogs"
    c.name # => "Joe Blogs"
    

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26. Plain Ol’ Ruby Objects
    class ConferenceDelegate
    :name
    end
    c = ConferenceDelegate.new
    c.name = "Joe Blogs"
    c.name # => "Joe Blogs"
    attr_accessor
    

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27. “Class” methods
    irb:> c.class.private_methods
    => ["abort", "alias_method", "at_exit", "attr",
    "attr_accessor", "attr_reader", "attr_writer",
    "binding", "block_given?", "define_method",
    "eval", "exec", "exit", "extended", "fail",
    "fork", "getc", "gets", "global_variables",
    "include", "included", "irb_binding", "lambda",
    "load", "local_variables", "private", "proc",
    "protected", "public", "puts", "raise",
    "remove_class_variable", "remove_const",
    "remove_instance_variable", "remove_method",
    "require", "sleep", "split", "sprintf", "srand",
    "sub", "sub!", "syscall", "system", "test",
    "throw", "undef_method", "untrace_var", "warn"]
    

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28. Ruby Hacker; Know Thy-self
    class ConferenceDelegate
    attr_accessor :name
    end
    # self == ConferenceDelegate
    

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29. Ruby Hacker; Know Thy-self
    class ConferenceDelegate
    def self.do_something
    “OK”
    end
    do_something
    end
    # => “OK”
    

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30. Ruby Hacker; Know Thy-self
    class ConferenceDelegate
    def self.has_name
    attr_accessor :name
    end
    has_name
    end
    t = ConferenceDelegate.new
    t.name = “Joe Blogs”
    

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31. Another module
    module Personable
    def has_person_attributes
    attr_accessor :first_name
    attr_accessor :last_name
    end
    end
    

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32. Adding “class” methods
    class RubyGuru
    extend Personable
    end
    has_person_attributes
    g = RubyGuru.new
    g.first_name = “Dave”
    g.last_name = “Thomas”
    

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33. Specifying Behaviour in Rails
    class SuperModel < ActiveRecord::Base
    validates_presence_of :rich_boyfriend
    validates_size_of :entourage,
    :minimum => 20
    has_many :vices, :through => :boyfriends
    end
    class FootballersWife < ActiveRecord::Base
    validates_presence_of :rich_boyfriend
    validates_size_of :entourage,
    :minimum => 5
    has_many :vices, :through => :boyfriends
    end
    

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34. Bundling behaviour
    module ModelValidation
    def acts_as_glamourous(size)
    validates_presence_of :rich_boyfriend
    validates_size_of :entourage,
    :minimum => size
    has_many :vices, :through => :boyfriends
    end
    end
    # ... add this method to the target class
    ActiveRecord::Base.send(:extend,
    ModelValidation)
    

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35. Our own ActiveRecord behaviour
    class Celebrity < AR::Base
    acts_as_glamourous(50)
    end
    

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36. Class and instance behaviour
    module GlamourPlugin
    def acts_as_glamourous(size)
    validates_presence_of :rich_boyfriend
    # etc...
    end
    end
    

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37. Class and instance behaviour
    module GlamourPlugin
    def acts_as_glamourous
    validates_presence_of :rich_boyfriend
    # etc...
    end
    module InstanceBehaviour
    def react_to(other_person)
    if other_person.is_a?(Paparazzo)
    other_person.destroy
    end
    end
    end
    end
    

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38. Class and instance behaviour
    module GlamourPlugin
    def acts_as_glamourous
    validates_presence_of :rich_boyfriend
    # etc...
    include GlamourPlugin::InstanceBehaviour
    end
    module InstanceBehaviour
    def react_to(other_person)
    if other_person.is_a?(Paparazzo)
    other_person.destroy
    end
    end
    end
    end
    

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39. Our plugin in action
    class Diva < ActiveRecord::Base
    acts_as_glamourous
    end
    dude = Paparazzo.create(:lens => "Huge")
    Paparazzo.count # => 1
    starlet = Diva.new(:name => "Britney",
    :entourage => 873,
    :quirk => "No hair")
    starlet.react_to(dude)
    Paparazzo.count # => 0
    

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40. Rails
    Patching
    Rails
    

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41. Rails
    Patching
    Rails
    HACKING
    

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42. changing the behaviour
    of existing classes
    

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43. Changing existing behaviour
    # replacing implementation
    # via inheritance
    class Thing < Object
    def object_id
    @my_custom_value
    end
    end
    

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44. Inheritance is not
    always possible
    ActiveRecord::Base
    ActionController::Base
    ActionView::Base
    Dependencies ActionMailer::Base
    Routing
    DispatchingAssociations
    ... and more...
    

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45. changing the
    implementation of
    existing methods
    

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46. Ruby classes are always open
    class ActiveRecord::Base
    def count
    execute("SELECT COUNT(*)
    FROM #{table_name}") / 2
    end
    end
    

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47. Aliasing methods
    class ActiveRecord::Base
    end
    alias_method :__count, :count
    def count
    __count / 2
    end
    

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48. Method chains with Rails
    class FunkyBass
    def play_it
    "Funky"
    end
    end
    bass = FunkyBass.new
    bass.play_it # => "Funky"
    

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49. Method chains - new behaviour
    module Soul
    def play_it_with_soul
    "Smooth & " +
    play_it_without_soul
    end
    end
    

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50. How alias_method_chain works
    class FunkyBass
    include Soul
    alias_method_chain :play_it, :soul
    end
    alias_method :play_it_without_soul,
    :play_it
    alias_method :play_it,
    :play_it_with_soul
    # underneath the hood:
    

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51. Adding the new functionality
    class FunkyBass
    include Soul
    alias_method_chain :play_it,
    :soul
    end
    bass.play_it
    # => "Smooth & Funky"
    

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52. Method chains in action
    class ActiveRecord::Base
    def count_with_fixes
    return count_without_fixes + 1
    end
    alias_method_chain :count, :fixes
    end
    MyModel.count # calls new method
    

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53. Patching Rails’ Dependencies
    class Dependencies
    def require_or_load(file_name)
    # load the file from the normal
    # places, i.e. $LOAD_PATH
    end
    end
    

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54. New plugin-loading behaviour
    module LoadingFromPlugins
    # we want to replace Rails’ default loading
    # behaviour with this
    def require_or_load_with_plugins(file_name)
    if file_exists_in_plugin(file_name)
    load_file_from_plugin(file_name)
    else
    require_or_load_without_plugins(file_name)
    end
    end
    end
    

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55. Injecting the new behaviour
    module LoadingFromPlugins
    def require_or_load_with_plugins(file_name)
    if file_exists_in_plugins(file_name)
    load_file_from_plugin(file_name)
    else
    require_or_load_without_plugins(file_name)
    end
    end
    end
    def self.included(base)
    base.send(:alias_method_chain,
    :require_or_load, :plugins)
    end
    Dependencies.send(:include, LoadingFromPlugins)
    

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