Rails 中的多级关联

Question

我正在创建一个应用程序来跟踪足球队整个赛季的情况。但我坚持数据库的设计。一场比赛有主队和客队。我创建了一个固定模型，它有两个外键 - home_team 和away_team，但我无法让关联正常工作。有什么想法吗？每场比赛都属于一个联赛。

Answer 1

简单的答案是：

class Fixture < ActiveRecord::Base
  belongs_to :home_team, :class_name => "Team", :foreign_key => :home_team
  belongs_to :away_team, :class_name => "Team", :foreign_key => :away_team
end

class Team < ActiveRecord::Base
  has_many :fixtures
end

但这并不好，因为 Team.fixtures 不起作用。

class Team < ActiveRecord::Base
  has_many :home_fixtures, :class_name => "Fixtures", :foreign_key => :home_team
  has_many :away_fixtures, :class_name => "Fixtures", :foreign_key => :away_team
end

会给你两个集合……但是聚合它们必须在 ruby​​ 中进行，这会很麻烦。

class Team < ActiveRecord::Base
  def fixtures(*args)
    home_fixtures.all(*args) + away_fixtures.all(*args)
  end
end

这也有它的问题，排序和限制将全部搞砸（呵呵，双关语，谁知道呢？）。

class Team < ActiveRecord::Base
  has_many :fixtures, :finder_sql => 'SELECT * FROM fixtures where (home_team = #{id} or away_team = #{id})'
  has_many :home_fixtures, :class_name => "Fixtures", :foreign_key => :home_team
  has_many :away_fixtures, :class_name => "Fixtures", :foreign_key => :away_team
end

这很丑陋，但可能可行。 finder_sql 似乎可以满足需要。

另一种选择是使用named_scope：

class Fixture < ActiveRecord::Base
  named_scope :for_team_id, lambda{|team_id| {:conditions => ['(home_team = ? or away_team = ?)', team_id, team_id]} }
  belongs_to :home_team, :class_name => "Team", :foreign_key => :home_team
  belongs_to :away_team, :class_name => "Team", :foreign_key => :away_team
end

class Team < ActiveRecord::Base
  def fixtures
    Fixtures.for_team_id(id)
  end
end

最后一个解决方案是我坚持使用的解决方案，因为它是一个作用域，它的行为非常类似于只读关联，并防止可能出现的 :finder_sql 怪异行为。发生在更远的地方（我的意思是真的吗？它怎么知道如何合并更多条件？它有时会执行子查询，子查询？这只是这里不需要的？）。

希望这有帮助。

Answer 2

假设您有一个 Team 模型和一个 Something 模型，后者将具有 home_team_id 和 away_team_id。

class Something < ActiveRecord::Base
  belongs_to :home_team, :class_name => 'Team'
  belongs_to :away_team, :class_name => 'Team'

您将把它们称为 something.away_team 和 something.home_team。