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#注意外键在person上people: id; name; whatever-you-need friendships: id; person_id; friend_id class Person < ActiveRecord::Base    has_many :friendships, :foreign_key => "person_id",       :class_name => "Friendship"    has_many :friends, :through => :friendships end class Friendship < ActiveRecord::Base    belongs_to :person, :foreign_key => "person_id", :class_name => "Person"    belongs_to :friend, :foreign_key => "friend_id", :class_name => "Person"  end

ActiveRecord::Schema.define(:version => 20100410040453) do   create_table "people", :force => true do |t|     t.string   "name"     t.string   "sex"     t.integer  "father_id"     t.integer  "mother_id"     t.datetime "created_at"     t.datetime "updated_at"   end end

#这里就要看source的应用，是用来关联belongs_to的 class Person < ActiveRecord::Base   self.inheritance_column = "sex"   belongs_to :father, :class_name => "Person"   belongs_to :mother, :class_name => "Person" end class Male < Person   has_many :children, :class_name => "Person", :foreign_key => :father_id   has_many :babymommas, :through => :children, :source => :mother, :uniq => true end class Female < Person   has_many :children, :class_name => "Person", :foreign_key => :mother_id   has_many :babydaddies, :through => :children, :source => :father, :uniq => true end

create_table :users do |t|   t.column :login, :string, :null => false end create_table :relationships, :id => false do |t|   t.column "user_id",  :integer, :null => false   t.column "buddy_id", :integer, :null => false end

describe User do   before :each do     %w(u v w).each do |x|       eval "@#{x} = User.new; @#{x}.login = '#{x}'; @#{x}.save"     end   end   it "can be linked with another user" do     @u.linked_to << @v     @u.linked_to.should include(@v)     u2 = User.find_by_id @u.id     u2.linked_to.should include(@v)   end   it "can find users that linked to it" do     @u.linked_to << @v     @v.linked_from.should include(@u)   end   it "can see friends of its friends" do     @u.linked_to << @v     @v.linked_to << @w     @u.linked_to[0].linked_to.should include(@w)     @w.linked_from[0].linked_from.should include(@u)   end   it "has a list of all users linked to and from it" do     @u.linked_to << @v     @v.linked_to << @w     @v.linked.should include(@u)     @v.linked.should include(@w)   end end

class Relationship < ActiveRecord::Base   belongs_to :user,     :class_name => 'User', :foreign_key => 'user_id'   belongs_to :buddy,     :class_name => 'User', :foreign_key => 'buddy_id' end class User < ActiveRecord::Base   has_many :relations_to,     :foreign_key => 'user_id',  :class_name => 'Relationship'   has_many :relations_from,     :foreign_key => 'buddy_id', :class_name => 'Relationship'                               has_many :linked_to,     :through => :relations_to,   :source => :buddy   has_many :linked_from,     :through => :relations_from, :source => :user end

select user_id, buddy_id     from relationships   union   select buddy_id, user_id      from relationships def linked   User.find_by_sql "     select *     from users     where id in (       select buddy_id       from buddies       where user_id = #{id})" end

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