Ruby on Rails ActiveRecord"has_many:through"唯一性验证

Question

目前我每次检查都会插入一个新关系,如果它不存在:

unless Relationship.exists?(:entry_id => entry.id, :tag_id => tag.id)

我如何在关系模型中实现这样的验证,以便它不允许在同一条目和标记之间有多个关系？

Answer 1

class Relationship < ActiveRecord::Base
  belongs_to :entry
  belongs_to :tag
  validates :tag_id, :uniqueness => { :scope => :entry_id }
end

Answer 2

假设您的模型看起来像这样:

class Entry < ActiveRecord::Base
  has_many :relationships
  has_many :tags, :through => :relationships
end

class Tag < ActiveRecord::Base
  has_many :relationships
  has_many :entries, :through => :relationships
end

class Relationship < ActiveRecord::Base
  belongs_to :entry
  belongs_to :tag
end

您可以为Relationship连接模型添加唯一验证:

validates_uniqueness_of :tag_id, :scope => :entry_id

该validates_uniqueness_of方法将确保关系尚不存在,并且该:scope选项将匹配范围限定为给定列.rails通过此验证生成的SQL将如下所示:

SELECT `relationships`.id
FROM `relationships`
WHERE (`relationships`.`tag_id` = <tag id> AND `relationships`.`entry_id` = <entry id>)
LIMIT 1

(您会注意到它与您明确使用的SQL生成的SQL基本相同Relationship.exists?(:entry_id => entry.id, :tag_id => tag.id)),如果找到记录,验证将失败.

同样,与您要验证唯一性的任何情况一样,请确保tag_id, entry_id您的relationships表中有唯一键.有关详细信息,请参阅此文章以及我上面链接的API页面的"并发性和完整性".