在Rails插件中扩展核心类无法正常工作

Question

Inside of #PLUGIN/lib/#PLUGIN I inserted the following:

String.class_eval do
  def to_squawk
    "squawk! #{self}".strip
  end
end

And when I try to test in Rails Console. I get the following error: NoMethodError: undefined method to_squawk' for "test string":String`

Loading development environment (Rails 4.2.0.beta2)
2.1.3 :001 > "test string".to_squawk
NoMethodError: undefined method `to_squawk' for "test string":String
  from (irb):1
  from /Users/fab/.rvm/gems/ruby-2.1.3/gems/railties-4.2.0.beta2/lib/rails/commands/console.rb:110:in `start'
  from /Users/fab/.rvm/gems/ruby-2.1.3/gems/railties-4.2.0.beta2/lib/rails/commands/console.rb:9:in `start'
  from /Users/fab/.rvm/gems/ruby-2.1.3/gems/railties-4.2.0.beta2/lib/rails/commands/commands_tasks.rb:68:in `console'
  from /Users/fab/.rvm/gems/ruby-2.1.3/gems/railties-4.2.0.beta2/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
  from /Users/fab/.rvm/gems/ruby-2.1.3/gems/railties-4.2.0.beta2/lib/rails/commands.rb:17:in `<top (required)>'
  from bin/rails:4:in `require'
  from bin/rails:4:in `<main>'

I am following the tutorial here: http://guides.rubyonrails.org/plugins.html

Does anyone know what the problem is here?

Answer 1

This is the code in the Rails Guide:

String.class_eval do
  def to_squawk
    "squawk! #{self}".strip
  end
end

This is your code:

String.class_eval do
  def self.to_squawk
    "squawk! #{self}".strip
  end
end

See the difference? The Guide's code defines an instance method that instances of String respond to (ie "foo".to_squawk # => "squawk! foo" ), but your code with its stray self defines a class method that the String class responds to (ie String.to_squawk # => "squawk! String" ).