has_many ：通过多个 has_one 关系？

我正在为我们的教会编写一个 Rails 指导计划（我对 Rails 还很陌生）。

我需要对此进行建模..

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

所以基本上一个对象“兄弟姐妹”需要映射来自父亲和母亲的所有孩子，不包括对象本身。

这可能吗？

Thanks

Daniel

有趣的是，看似简单的问题却有复杂的答案。在这种情况下，实现反射性父/子关系相当简单，但添加父/母和兄弟姐妹关系会产生一些麻烦。

首先，我们创建表来保存父子关系。关系有两个外键，都指向联系人：

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

在关系模型中，我们将父亲和母亲指向联系人：

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

并在 Contact 中定义逆关联：

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

现在可以创建关系：

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

这并不是很好，我们真正想要的是在一次调用中建立关系：

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

所以我们可以这样做：

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

要查找联系人的子级，请向联系人添加范围和（为了方便起见）实例方法：

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。我们可以利用 Contact.children 方法并操纵结果：

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

这不是最佳的，因为father.children和mother.children会重叠（因此需要uniq），并且可以通过制定必要的 SQL（留作练习:)）来更有效地完成，但请记住self.father.children and self.mother.children如果是同父异母的兄弟姐妹（同父异母），则不会重叠，并且联系人可能没有父亲或母亲。

以下是完整的型号和一些规格：

# 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