Question

我正在使用Ruby on Rails 3，我希望通过关系类为2个类中的更多类设置多对多关联。

例如，我有：

class RelationshipGroup < ActiveRecord::Base # This is the relationship class

  # Maybe the Schema Information should be the following:
  #
  # Table name: RelationshipGroup
  #
  # id    : integer
  # dog_id: integer
  # cat_id: integer
  # cow_id: integer

  ...
end 

class Dog < ActiveRecord::Base
  ...
end 

class Cat < ActiveRecord::Base
  ...
end 

class Cow < ActiveRecord::Base
  ...
end

在上面的示例中，我想设置这些关联，以便可以使用RelationshipGroup类进行搜索，以便我可以检索属于某个组的所有动物。我知道如何将has_many :through关联用于2个类，但不能用于2个以上。因此，可以实现（可能我必须使用关联扩展或类方法来达到？）？如果是这样，我如何编写上述示例的代码？

Answer 1

您可以通过连接表使用多态关联。

class RelationshipGroup < ActiveRecord::Base # This is the relationship class
  has_many :memberships
  has_many :members, :through => :memberships
end 

class Membership
  #fields - relationship_group_id, member_id, member_type
  belongs_to :relationship_group
  belongs_to :member, :polymorphic => true
end

class Dog < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships
end 

class Cat < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships    
end 

class Cow < ActiveRecord::Base
  has_many :memberships, :as => :member
  has_many :relationship_groups, :through => :memberships    
end

为了更进一步，通过将关联（都是相同的）移动到模块中来干掉它会很好：

#in lib/member.rb
module Member
  def self.included(base)
    base.class_eval do 
      has_many :memberships, :as => :member
      has_many :relationship_groups, :through => :memberships   
    end    
  end     
end 

class RelationshipGroup < ActiveRecord::Base # This is the relationship class
  has_many :memberships
  has_many :members, :through => :memberships
end 

class Membership
  #fields - relationship_group_id, member_id, member_type
  belongs_to :relationship_group
  belongs_to :member, :polymorphic => true
end

class Dog < ActiveRecord::Base
  include Member
end 

class Cat < ActiveRecord::Base
  include Member 
end 

class Cow < ActiveRecord::Base
  include Member
end

通过关系类为2个类中的更多类创建多对多关联

1 个答案: