ActiveRecord自带了三种数据结构关系:acts_as_tree、acts_as_list、acts_as_nested_set

1，tree.rb

module ActiveRecord
  module Acts
    module Tree

      def self.included(base)
        base.extend(ClassMethods)
      end

      module ClassMethods
        def acts_as_tree(options = {})
          configuration = { :foreign_key => "parent_id", :order => nil, :counter_cache => nil }
          configuration.update(options) if options.is_a?(Hash)

          belongs_to :parent, :class_name => name, :foreign_key => configuration[:foreign_key], :counter_cache => configuration[:counter_cache]
          has_many :children, :class_name => name, :foreign_key => configuration[:foreign_key], :order => configuration[:order], :dependent => :destroy

          class_eval <<-EOV
            include ActiveRecord::Acts::Tree::InstanceMethods

            def self.roots
              find(:all, :conditions => "#{configuration[:foreign_key]} IS NULL", :order => #{configuration[:order].nil? ? "nil" : %Q{"#{configuration[:order]}"}})
            end

            def self.root
              find(:first, :conditions => "#{configuration[:foreign_key]} IS NULL", :order => #{configuration[:order].nil? ? "nil" : %Q{"#{configuration[:order]}"}})
            end
          EOV
        end
      end

      module InstanceMethods
        def ancestors
          node, nodes = self, []
          nodes << node = node.parent while node.parent
          nodes
        end

        def root
          node = self
          node = node.parent while node.parent
          node
        end

        def siblings
          self_and_siblings - [self]
        end

        def self_and_siblings
          parent ? parent.children : self.class.roots
        end
      end
    end
  end
end

从上面的代码我们学习到如下几点:
1) acts_as_tree实际上定义了belongs_to :parent和has_many :children，也就是说我们可以通过@model.parent和@model.children得到父子对象
2) acts_as_tree的配置参数有:foreign_key、:order => nil和:counter_cache
3) 其中定义了roots和root这两个Class Methods
4) 其中还定义了ancestors、root、sliblings、self_and_siblings这些Instance Methods

2，list.rb

module ActiveRecord
  module Acts
    module List

      def self.included(base)
        base.extend(ClassMethods)
      end

      module ClassMethods
        def acts_as_list(options = {})
          configuration = { :column => "position", :scope => "1 = 1" }
          configuration.update(options) if options.is_a?(Hash)

          configuration[:scope] = "#{configuration[:scope]}_id".intern if configuration[:scope].is_a?(Symbol) && configuration[:scope].to_s !~ /_id$/
          
          if configuration[:scope].is_a?(Symbol)
            scope_condition_method = %(
              def scope_condition
                if #{configuration[:scope].to_s}.nil?
                  "#{configuration[:scope].to_s} IS NULL"
                else
                  "#{configuration[:scope].to_s} = \#{#{configuration[:scope].to_s}}"
                end
              end
            )
          else
            scope_condition_method = "def scope_condition() \"#{configuration[:scope]}\" end"
          end
          
          class_eval <<-EOV
            include ActiveRecord::Acts::List::InstanceMethods

            def acts_as_list_class
              ::#{self.name}
            end
            
            def position_column
              '#{configuration[:column]}'
            end
            
            #{scope_condition_method}
            
            after_destroy  :remove_from_list
            before_create  :add_to_list_bottom
          EOV
        end
      end

      module InstanceMethods
        def insert_at(position = 1)
          insert_at_position(position)
        end

        def move_lower
          return unless lower_item

          acts_as_list_class.transaction do
            lower_item.decrement_position
            increment_position
          end
        end

        def move_higher
          return unless higher_item

          acts_as_list_class.transaction do
            higher_item.increment_position
            decrement_position
          end
        end

        def move_to_bottom
          return unless in_list?
          acts_as_list_class.transaction do
            decrement_positions_on_lower_items
            assume_bottom_position
          end
        end
        
        def move_to_top
          return unless in_list?
          acts_as_list_class.transaction do
            increment_positions_on_higher_items
            assume_top_position
          end
        end
        
        def remove_from_list
          decrement_positions_on_lower_items if in_list?
        end

        def increment_position
          return unless in_list?
          update_attribute position_column, self.send(position_column).to_i + 1
        end
  
        def decrement_position
          return unless in_list?
          update_attribute position_column, self.send(position_column).to_i - 1
        end
  
        def first?
          return false unless in_list?
          self.send(position_column) == 1
        end
        
        def last?
          return false unless in_list?
          self.send(position_column) == bottom_position_in_list
        end

        def higher_item
          return nil unless in_list?
          acts_as_list_class.find(:first, :conditions =>
            "#{scope_condition} AND #{position_column} = #{(send(position_column).to_i - 1).to_s}"
          )
        end

        def lower_item
          return nil unless in_list?
          acts_as_list_class.find(:first, :conditions =>
            "#{scope_condition} AND #{position_column} = #{(send(position_column).to_i + 1).to_s}"
          )
        end

        def in_list?
          !send(position_column).nil?
        end

        private
          def add_to_list_top
            increment_positions_on_all_items
          end

          def add_to_list_bottom
            self[position_column] = bottom_position_in_list.to_i + 1
          end

          def scope_condition() "1" end

          def bottom_position_in_list(except = nil)
            item = bottom_item(except)
            item ? item.send(position_column) : 0
          end

          def bottom_item(except = nil)
            conditions = scope_condition
            conditions = "#{conditions} AND #{self.class.primary_key} != #{except.id}" if except
            acts_as_list_class.find(:first, :conditions => conditions, :order => "#{position_column} DESC")
          end

          def assume_bottom_position
            update_attribute(position_column, bottom_position_in_list(self).to_i + 1)
          end
  
          def assume_top_position
            update_attribute(position_column, 1)
          end

          def decrement_positions_on_higher_items(position)
            acts_as_list_class.update_all(
              "#{position_column} = (#{position_column} - 1)", "#{scope_condition} AND #{position_column} <= #{position}"
            )
          end

          def decrement_positions_on_lower_items
            return unless in_list?
            acts_as_list_class.update_all(
              "#{position_column} = (#{position_column} - 1)", "#{scope_condition} AND #{position_column} > #{send(position_column).to_i}"
            )
          end

          def increment_positions_on_higher_items
            return unless in_list?
            acts_as_list_class.update_all(
              "#{position_column} = (#{position_column} + 1)", "#{scope_condition} AND #{position_column} < #{send(position_column).to_i}"
            )
          end

          def increment_positions_on_lower_items(position)
            acts_as_list_class.update_all(
              "#{position_column} = (#{position_column} + 1)", "#{scope_condition} AND #{position_column} >= #{position}"
           )
          end

          def increment_positions_on_all_items
            acts_as_list_class.update_all(
              "#{position_column} = (#{position_column} + 1)",  "#{scope_condition}"
            )
          end

          def insert_at_position(position)
            remove_from_list
            increment_positions_on_lower_items(position)
            self.update_attribute(position_column, position)
          end
      end     
    end
  end
end

从上面的代码我们学习到，acts_as_list的配置参数有:column和:scope
1) :column默认名为position
2) :scope后的参数为symbol时，如果没有_id则加上_id后缀

class TodoList < ActiveRecord::Base
  has_many :todo_items, :order => "position"
end

class TodoItem < ActiveRecord::Base
  belongs_to :todo_list
  acts_as_list :scope => :todo_list
end

这样生成的scope_condition为"todo_list_id = #{todo_list_id}"
3) :scope后的参数为string时可以进行细粒度的限制

class TodoList < ActiveRecord::Base
  has_many :todo_items, :order => "position"
end

class TodoItem < ActiveRecord::Base
  belongs_to :todo_list
  acts_as_list :scope => 'todo_list_id = #{todo_list_id} AND completed = 0'
end

4) 实例方法很多，简单看看即可，不一一介绍了

3，nested_set.rb

module ActiveRecord
  module Acts
    module NestedSet

      def self.included(base)
        base.extend(ClassMethods)              
      end  

      module ClassMethods                      
        def acts_as_nested_set(options = {})
          configuration = { :parent_column => "parent_id", :left_column => "lft", :right_column => "rgt", :scope => "1 = 1" }
          
          configuration.update(options) if options.is_a?(Hash)
          
          configuration[:scope] = "#{configuration[:scope]}_id".intern if configuration[:scope].is_a?(Symbol) && configuration[:scope].to_s !~ /_id$/
          
          if configuration[:scope].is_a?(Symbol)
            scope_condition_method = %(
              def scope_condition
                if #{configuration[:scope].to_s}.nil?
                  "#{configuration[:scope].to_s} IS NULL"
                else
                  "#{configuration[:scope].to_s} = \#{#{configuration[:scope].to_s}}"
                end
              end
            )
          else
            scope_condition_method = "def scope_condition() \"#{configuration[:scope]}\" end"
          end
        
          class_eval <<-EOV
            include ActiveRecord::Acts::NestedSet::InstanceMethods

            #{scope_condition_method}
            
            def left_col_name() "#{configuration[:left_column]}" end

            def right_col_name() "#{configuration[:right_column]}" end
              
            def parent_column() "#{configuration[:parent_column]}" end

          EOV
        end
      end
      
      module InstanceMethods
        def root?
          parent_id = self[parent_column]
          (parent_id == 0 || parent_id.nil?) && (self[left_col_name] == 1) && (self[right_col_name] > self[left_col_name])
        end                                                                                             

        def child?                          
          parent_id = self[parent_column]
          !(parent_id == 0 || parent_id.nil?) && (self[left_col_name] > 1) && (self[right_col_name] > self[left_col_name])
        end     
        
        def unknown?
          !root? && !child?
        end

        def add_child( child )     
          self.reload
          child.reload

          if child.root?
            raise "Adding sub-tree isn\'t currently supported"
          else
            if ( (self[left_col_name] == nil) || (self[right_col_name] == nil) )
              self[left_col_name] = 1
              self[right_col_name] = 4
              
              return nil unless self.save
              
              child[parent_column] = self.id
              child[left_col_name] = 2
              child[right_col_name]= 3
              return child.save
            else
              child[parent_column] = self.id
              right_bound = self[right_col_name]
              child[left_col_name] = right_bound
              child[right_col_name] = right_bound + 1
              self[right_col_name] += 2
              self.class.base_class.transaction {
                self.class.base_class.update_all( "#{left_col_name} = (#{left_col_name} + 2)",  "#{scope_condition} AND #{left_col_name} >= #{right_bound}" )
                self.class.base_class.update_all( "#{right_col_name} = (#{right_col_name} + 2)",  "#{scope_condition} AND #{right_col_name} >= #{right_bound}" )
                self.save
                child.save
              }
            end
          end                                   
        end
                                   
        def children_count
          return (self[right_col_name] - self[left_col_name] - 1)/2
        end
                                                               
        def full_set
          self.class.base_class.find(:all, :conditions => "#{scope_condition} AND (#{left_col_name} BETWEEN #{self[left_col_name]} and #{self[right_col_name]})" )
        end

        def all_children
          self.class.base_class.find(:all, :conditions => "#{scope_condition} AND (#{left_col_name} > #{self[left_col_name]}) and (#{right_col_name} < #{self[right_col_name]})" )
        end
                                  
        def direct_children
          self.class.base_class.find(:all, :conditions => "#{scope_condition} and #{parent_column} = #{self.id}")
        end
                                      
        def before_destroy
          return if self[right_col_name].nil? || self[left_col_name].nil?
          dif = self[right_col_name] - self[left_col_name] + 1

          self.class.base_class.transaction {
            self.class.base_class.delete_all( "#{scope_condition} and #{left_col_name} > #{self[left_col_name]} and #{right_col_name} < #{self[right_col_name]}" )
            self.class.base_class.update_all( "#{left_col_name} = (#{left_col_name} - #{dif})",  "#{scope_condition} AND #{left_col_name} >= #{self[right_col_name]}" )
            self.class.base_class.update_all( "#{right_col_name} = (#{right_col_name} - #{dif} )",  "#{scope_condition} AND #{right_col_name} >= #{self[right_col_name]}" )
          }
        end
      end
    end
  end
end

我们可以得到如下几点:
1) acts_as_nested_set和acts_as_tree很类似，但是多一个特性是可以一条语句查询出所有的children

def all_children
  self.class.base_class.find(:all, :conditions => "#{scope_condition} AND (#{left_col_name} > #{self[left_col_name]}) and (#{right_col_name} < #{self[right_col_name]})" )
end

以及高效率的计算children_count

def children_count
  return (self[right_col_name] - self[left_col_name] - 1)/2
end

2) acts_as_nested_set的参数为:parent_column、:left_column、:right_column和:scope
3) 这种数据结构常用在threaded post system等场景

看了上述代码，相信大家可以自己动手写acts_as_xx数据结构了

BTW:对ActiveRecord源码的研究告一段落，还有query_cache/observer/xml_serialization/timestamp/calculations/migration等留待大家自己研究
接下来看看ActionController吧