has_many：通過多個has_one關系？

Question

我正在為我們的鐵路教堂寫一個導師計划（我仍然很喜歡鐵路）。

我需要對此進行建模..

contact
has_one :father, :class_name => "Contact"
has_one :mother, :class_name => "Contact"
has_many :children, :class_name => "Contact"
has_many :siblings, :through <Mother and Father>, :source => :children

所以基本上一個對象“兄弟姐妹”需要映射父親和母親的所有孩子，不包括對象本身。

這可能嗎？

謝謝

丹尼爾

Answer 1

有趣的是，看似簡單的問題可以得到復雜的答案。 在這種情況下，實現反身父/子關系相當簡單，但添加父/母和兄弟關系會產生一些曲折。

首先，我們創建表來保存父子關系。 關系有兩個外鍵，都指向聯系人：

create_table :contacts do |t|
  t.string :name
end

create_table :relationships do |t|
  t.integer :contact_id
  t.integer :relation_id
  t.string :relation_type
end

在關系模型中，我們將父親和母親指回聯系人：

class Relationship < ActiveRecord::Base
  belongs_to :contact
  belongs_to :father, :foreign_key => :relation_id, :class_name => "Contact",
  :conditions => { :relationships => { :relation_type => 'father'}}
  belongs_to :mother, :foreign_key => :relation_id, :class_name => "Contact",
  :conditions => { :relationships => { :relation_type => 'mother'}}
end

並在Contact中定義反向關聯：

class Contact < ActiveRecord::Base
  has_many :relationships, :dependent => :destroy
  has_one :father, :through => :relationships
  has_one :mother, :through => :relationships
end

現在可以創建一個關系：

@bart = Contact.create(:name=>"Bart")
@homer = Contact.create(:name=>"Homer")
@bart.relationships.build(:relation_type=>"father",:father=>@homer)
@bart.save!
@bart.father.should == @homer

這不是很好，我們真正想要的是在一次通話中建立關系：

class Contact < ActiveRecord::Base
  def build_father(father)
    relationships.build(:father=>father,:relation_type=>'father')
  end
end

所以我們可以這樣做：

@bart.build_father(@homer)
@bart.save!

要查找聯系人的子項，請向Contact和（為方便起見）實例方法添加范圍：

scope :children, lambda { |contact| joins(:relationships).\
  where(:relationships => { :relation_type => ['father','mother']}) }

def children
  self.class.children(self)
end

Contact.children(@homer) # => [Contact name: "Bart")]
@homer.children # => [Contact name: "Bart")]

兄弟姐妹是棘手的部分。 我們可以利用Contact.children方法並操縱結果：

def siblings
  ((self.father ? self.father.children : []) +
   (self.mother ? self.mother.children : [])
   ).uniq - [self]
end

這是非最優的，因為父親和孩子將會重疊（因此需要uniq ），並且可以通過制定必要的SQL（左側作為練習:)來更有效地完成，但要記住， self.father.children和self.mother.children在半兄弟姐妹（同一個父親，不同的母親）的情況下不會重疊，並且聯系人可能沒有父親或母親。

以下是完整的型號和一些規格：

# app/models/contact.rb
class Contact < ActiveRecord::Base
  has_many :relationships, :dependent => :destroy
  has_one :father, :through => :relationships
  has_one :mother, :through => :relationships

  scope :children, lambda { |contact| joins(:relationships).\
    where(:relationships => { :relation_type => ['father','mother']}) }

  def build_father(father)
    # TODO figure out how to get ActiveRecord to create this method for us
    # TODO failing that, figure out how to build father without passing in relation_type
    relationships.build(:father=>father,:relation_type=>'father')
  end

  def build_mother(mother)
    relationships.build(:mother=>mother,:relation_type=>'mother')
  end

  def children
    self.class.children(self)
  end

  def siblings
    ((self.father ? self.father.children : []) +
     (self.mother ? self.mother.children : [])
     ).uniq - [self]
  end
end

# app/models/relationship.rb
class Relationship < ActiveRecord::Base
  belongs_to :contact
  belongs_to :father, :foreign_key => :relation_id, :class_name => "Contact",
  :conditions => { :relationships => { :relation_type => 'father'}}
  belongs_to :mother, :foreign_key => :relation_id, :class_name => "Contact",
  :conditions => { :relationships => { :relation_type => 'mother'}}
end

# spec/models/contact.rb
require 'spec_helper'

describe Contact do
  before(:each) do
    @bart = Contact.create(:name=>"Bart")
    @homer = Contact.create(:name=>"Homer")
    @marge = Contact.create(:name=>"Marge")
    @lisa = Contact.create(:name=>"Lisa")
  end

  it "has a father" do
    @bart.relationships.build(:relation_type=>"father",:father=>@homer)
    @bart.save!
    @bart.father.should == @homer
    @bart.mother.should be_nil
  end

  it "can build_father" do
    @bart.build_father(@homer)
    @bart.save!
    @bart.father.should == @homer
  end

  it "has a mother" do
    @bart.relationships.build(:relation_type=>"mother",:father=>@marge)
    @bart.save!
    @bart.mother.should == @marge
    @bart.father.should be_nil
  end

  it "can build_mother" do
    @bart.build_mother(@marge)
    @bart.save!
    @bart.mother.should == @marge
  end

  it "has children" do
    @bart.build_father(@homer)
    @bart.build_mother(@marge)
    @bart.save!
    Contact.children(@homer).should include(@bart)
    Contact.children(@marge).should include(@bart)
    @homer.children.should include(@bart)
    @marge.children.should include(@bart)
  end

  it "has siblings" do
    @bart.build_father(@homer)
    @bart.build_mother(@marge)
    @bart.save!
    @lisa.build_father(@homer)
    @lisa.build_mother(@marge)
    @lisa.save!
    @bart.siblings.should == [@lisa]
    @lisa.siblings.should == [@bart]
    @bart.siblings.should_not include(@bart)
    @lisa.siblings.should_not include(@lisa)
  end

  it "doesn't choke on nil father/mother" do
    @bart.siblings.should be_empty
  end
end

Answer 2

我完全同意zetetic。 這個問題看起來比答案簡單得多，我們幾乎無能為力。 我會添加我的20c。
表：

    create_table :contacts do |t|
      t.string :name
      t.string :gender
    end
    create_table :relations, :id => false do |t|
      t.integer :parent_id
      t.integer :child_id
    end

表關系沒有相應的模型。

class Contact < ActiveRecord::Base
  has_and_belongs_to_many :parents,
    :class_name => 'Contact',
    :join_table => 'relations',
    :foreign_key => 'child_id',
    :association_foreign_key => 'parent_id'

  has_and_belongs_to_many :children,
    :class_name => 'Contact',
    :join_table => 'relations',
    :foreign_key => 'parent_id',
    :association_foreign_key => 'child_id'

  def siblings
    result = self.parents.reduce [] {|children, p| children.concat  p.children}
    result.uniq.reject {|c| c == self}
  end

  def father
    parents.where(:gender => 'm').first
  end

  def mother
    parents.where(:gender => 'f').first
  end
end  

現在我們定期進行Rails分析。 所以我們可以

alice.parents << bob
alice.save

bob.chidren << cindy
bob.save

alice.parents.create(Contact.create(:name => 'Teresa', :gender => 'f')

以及所有類似的東西。

Answer 3

  has_and_belongs_to_many :parents,
    :class_name => 'Contact',
    :join_table => 'relations',
    :foreign_key => 'child_id',
    :association_foreign_key => 'parent_id',
    :delete_sql = 'DELETE FROM relations WHERE child_id = #{id}'

  has_and_belongs_to_many :children,
    :class_name => 'Contact',
    :join_table => 'relations',
    :foreign_key => 'parent_id',
    :association_foreign_key => 'child_id',
    :delete_sql = 'DELETE FROM relations WHERE parent_id = #{id}'

我使用了這個例子，但不得不添加：delete_sql來清理關系記錄。 起初我在字符串周圍使用雙引號但發現導致錯誤。 切換到單引號有效。