database-design - 我可以用多个具体的超级表来完成这个吗？

Question

我有两个可以评论和评分的表格，“内容”和“个人资料”。我已经研究过使用多态关联并决定反对它。如果我要使用多态关联，表“评分”和“评论”都将具有此功能。

这可以通过具体的超表实现来实现吗？如果是这样，我该怎么做？

Answer 1

You're mixing up columns on the join. It should be this:

  find(self.id, :joins => "JOIN commentables cm ON (profiles.commentable_id = cm.id) 
  LEFT OUTER JOIN comments c ON (cm.id = c.commentable_id)")

Answer 2

尝试这个：

class Commentable < ActiveRecord::Base
  has_many :comments
  has_one :profile, :content
end

class Comment < ActiveRecord::Base
  belongs_to :commentable
end

class Content < ActiveRecord::Base
  belongs_to :commentable
end

class Profile < ActiveRecord::Base
  belongs_to :commentable
end

这使您能够（脚本/控制台）：

# every time you create a profile, link it to the 
# commentable row associated to this profile instance
cid = Commentable.create().id
p = Profile.create(:name => "Inspector Gadget", 
                   :commentable_id => cid)

# when creating the comment, assign the commentable_id 
# to link it to a Profile comment
Comment.new do |c|
  c.body = "go go gadget"
  c.email = "Inspector.Gadget@example.com"
  c.commentable_id = p.commentable_id
  c.save!
end

要从配置文件中检索数据库中的评论，请使用以下技巧：

# Profile.rb

# instead of doing a has many through, 
# we can just use instance method with custom join.
def comments
  find(self.id, :joins => "JOIN commentables cm ON (profiles.id = cm.id) 
  LEFT OUTER JOIN comments c ON (cm.id = c.commentable_id)")
end

未经测试的代码！

有关更多详细信息和说明，请参见此处，为什么在多态关联中不能有外键？

Answer 3

如果我错了，请纠正我，但你不能在 Profiles.rb 中这样做吗

def comments
  Comments.find(:all, :conditions => ["commentable_id = ?", self.commentable_id])
end