分析ActiveRecord使用method_missing和respond_to?实现动态方法

method_missing经常用来写Ruby的元编程。例如，如果你的UserModel有个email的属性，你就可以通过

User.find_by_email('[e-mail]')

来查找，这是如果User并没有定义这个方法，那么ActiveRecord::Base就会处理这样的请求。后面我们会具体分析这过程的逻辑和实现，并学习处理。

respond_to?在实现动态编程中也经常会用到，通常我们在使用respond之前判断是否有respond。
那么我们具体看看实现：
假设我们有Legislator类，我们将实现从find_by_first_name('John') 到find(:first_name => 'John')的动态逻辑
如下：

class Legislator
  # Pretend this is a real implementation
  def find(conditions = {})
  end
  
  # Define on self, since it's  a class method
  def self.method_missing(method_sym, *arguments, &block)
    # the first argument is a Symbol, so you need to_s it if you want to pattern match
    if method_sym.to_s =~ /^find_by_(.*)$/
      find($1.to_sym => arguments.first)
    else
      super
    end
  end
end

按照道理来说，这是通过样的逻辑，只是Legislator.respond_to?(:find_by_first_name)会返回false那么我们需要respond_to?来支持。

class Legislator
  # ommitted
  
  # It's important to know Object defines respond_to to take two parameters: the method to check, and whether to include private methods
  # http://www.ruby-doc.org/core/classes/Object.html#M000333
  def self.respond_to?(method_sym, include_private = false)
    if method_sym.to_s =~ /^find_by_(.*)$/
      true
    else
      super
    end
  end
end

那么现在有一个问题，就是我们的重复了。这就很不DRY。所以，我们可以和ActiveRecord的学习，看看是怎么处理重复的问题的。实际上ActiveRecord把逻辑封装在了ActiveRecord::DynamicFinderMatch以便不会在method_missing 和respond_to?重复代码

class LegislatorDynamicFinderMatch
  attr_accessor :attribute
  def initialize(method_sym)
    if method_sym.to_s =~ /^find_by_(.*)$/
      @attribute = $1.to_sym
    end
  end
  
  def match?
    @attribute != nil
  end
end

class Legislator
  def self.method_missing(method_sym, *arguments, &block)
    match = LegislatorDynamicFinderMatch.new(method_sym)
    if match.match?
      find(match.attribute => arguments.first)
    else
      super
    end
  end

  def self.respond_to?(method_sym, include_private = false)
    if LegislatorDynamicFinderMatch.new(method_sym).match?
      true
    else
      super
    end
  end
end

method_missing的缓冲
显然method missing效率不好，那么太多的method missing一定导致很慢。所以另外一个我们可以学习ActiveRecord的地方是我们可以在定义method missing的同时发送到正定义的方法，如下：

class Legislator    
  def self.method_missing(method_sym, *arguments, &block)
    match = LegislatorDynamicFinderMatch.new(method_sym)
    if match.match?
      define_dynamic_finder(method_sym, match.attribute)
      send(method_sym, arguments.first)
    else
      super
    end
  end
  
  protected
  
  def self.define_dynamic_finder(finder, attribute)
    class_eval <<-RUBY
      def self.#{finder}(#{attribute})        # def self.find_by_first_name(first_name)
        find(:#{attribute} => #{attribute})   #   find(:first_name => first_name)
      end                                     # end
    RUBY
  end
end

测试
创建LegislatorDynamicFinderMatch来测试逻辑，下面是RSpec的例子：

describe LegislatorDynamicFinderMatch do
  describe 'find_by_first_name' do
    before do
      @match = LegislatorDynamicFinderMatch.new(:find_by_first_name)
    end
      
    it 'should have attribute :first_name' do
      @match.attribute.should == :first_name
    end
    
    it 'should be a match' do
      @match.should be_a_match
    end
  end
  
  describe 'zomg' do
    before do
      @match = LegislatorDynamicFinderMatch(:zomg)
    end
    
    it 'should have nil attribute' do
      @match.attribute.should be_nil
    end
    
    it 'should not be a match' do
      @match.should_not be_a_match
    end
  end
end

当然，如果你的动态实习需要一些输入的话，很难免你需要用到RSpec，如下：

describe Legislator, 'dynamic find_by_first_name' do
  it 'should call find(:first_name => first_name)' do
    Legislator.should_receive(:find).with(:first_name => 'John')
    
    Legislator.find_by_first_name('John')
  end
end

总之，如果你正在写动态方法你应该考虑respond_to?