我希望能够创建元类方法例如，我的网站上有一个自动完成机制来处理select_标记。
以下是我希望如何构建我的选择选项：
f.select(:subjects, options_from_collection_for_select(Subject.autocomplete_for_subject_title, :id, :title))
所以我的autocomplete_for_subject_title类方法可以返回for now subject.all
我考虑的是一个模块，而不是为给定的模型定义一个类方法，如下所示：
主题.rb
class Subject
include SimpleAutocomplete
field :title, :type => String
autocomplete_for :subject, :title
end
lib/simple_自动完成.rb
module SimpleAutocomplete
def self.included(base)
base.class_eval do |klass|
extend ClassMethods
end
end
module ClassMethods
def autocomplete_for(object, method, options = {}, &block)
(class << object; object; end).instance_eval { define_method "autocomplete_for_#{object}_#{method}" do
puts "It works!"
end
}
end
end
end
但当我启动rails服务器时，我得到的是：
simple_autocomplete.rb:13:in `autocomplete_for': can't define singleton (TypeError)
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/app/models/subject.rb:20:in `<class:Subject>'
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/app/models/subject.rb:1:in `<top (required)>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:454:in `load'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:454:in `block in load_file'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:596:in `new_constants_in'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:453:in `load_file'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:340:in `require_or_load'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:300:in `depend_on'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:216:in `require_dependency'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/rails/mongoid.rb:55:in `load_model'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/rails/mongoid.rb:18:in `block (2 levels) in load_models'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/rails/mongoid.rb:17:in `each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/rails/mongoid.rb:17:in `block in load_models'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/paths.rb:102:in `block in each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/paths.rb:102:in `each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/paths.rb:102:in `each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/rails/mongoid.rb:16:in `load_models'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/mongoid-2.0.2/lib/mongoid/railtie.rb:86:in `block (2 levels) in <class:Railtie>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/callbacks.rb:419:in `_run_prepare_callbacks'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/callbacks.rb:40:in `initialize'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:33:in `new'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:33:in `build'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:79:in `block in build'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:79:in `each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:79:in `inject'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/actionpack-3.0.4/lib/action_dispatch/middleware/stack.rb:79:in `build'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/application.rb:162:in `app'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/application/finisher.rb:35:in `block in <module:Finisher>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/initializable.rb:25:in `instance_exec'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/initializable.rb:25:in `run'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/initializable.rb:50:in `block in run_initializers'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/initializable.rb:49:in `each'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/initializable.rb:49:in `run_initializers'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/application.rb:134:in `initialize!'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/application.rb:77:in `method_missing'
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/config/environment.rb:10:in `<top (required)>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:239:in `require'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:239:in `block in require'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:225:in `block in load_dependency'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:596:in `new_constants_in'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:225:in `load_dependency'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.4/lib/active_support/dependencies.rb:239:in `require'
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/config.ru:3:in `block in <main>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/builder.rb:46:in `instance_eval'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/builder.rb:46:in `initialize'
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/config.ru:1:in `new'
from /Users/pierrelouisgottfrois/Work/teamento_beta_git/config.ru:1:in `<main>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/builder.rb:35:in `eval'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/builder.rb:35:in `parse_file'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/server.rb:162:in `app'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/server.rb:248:in `wrapped_app'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/rack-1.2.1/lib/rack/server.rb:213:in `start'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/commands/server.rb:65:in `start'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/commands.rb:30:in `block in <top (required)>'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/commands.rb:27:in `tap'
from /Users/pierrelouisgottfrois/.rvm/gems/ruby-1.9.2-p180/gems/railties-3.0.4/lib/rails/commands.rb:27:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
我已经阅读了这篇博文开始：http://blog.jayfields.com/2007/10/ruby-defining-class-methods.html
你知道吗？
最佳答案
您试图通过object调用class << object的单例类，但是object是符号:subject，我认为这不是您想要的。
我不知道你到底想要什么，介意吧，但那也许能帮你弄清楚。