由于关系，DataMapper 无法删除记录

我使用 Torrent 和 Tag 设置了多对 DataMapper/MySQL，如下所示：

class Torrent
  include DataMapper::Resource

  property :id,          Serial
  property :name,        String
  property :magnet,      Text
  property :created_at,  DateTime

  has n, :tags, :through => Resource
end

class Tag
  include DataMapper::Resource

  property :id,      Serial
  property :name,    String
  property :hits,    Integer

  has n, :torrents, :through => Resource
end

然而，当试图破坏种子时，通过Torrent.first.destroy或类似的东西，DataMapper 返回false.

我尝试过直接 SQL 查询，例如delete from torrents where name like '%ubuntu%'，由于 MySQL 错误 1451 而失败：

ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constraint fails (`brightswipe`.`tag_torrents`, CONSTRAINT `tag_torrents_torrent_fk` FOREIGN KEY (`torrent_id`) REFERENCES `torrents` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION)

我认为有一些 DataMapper 设置，在删除 torrent 时我可以：

1. 删除标签关联
2. 删除种子

当删除标签时，我可以：

1. 从具有该标签的所有种子中删除标签关联
2. 删除标签

我该怎么办？

尝试使用此插件自动管理关系：

https://github.com/datamapper/dm-constraints https://github.com/datamapper/dm-constraints

这将允许您销毁 M:M assocs，尽管您必须手动清理 assocs 表：

class Tag
  ...
  has n, :torrents, :through => Resource, :constraint => :skip

class Torrent
  ...
  has n, :tags, :through => Resource, :constraint => :skip

另外一个选择是手动从 assocs 表中删除关系，然后您可以毫无问题地删除项目，因为您通过从 assocs 表中删除相应条目来破坏关系。

基本示例：

tr = Torrent.create
tg = Tag.create

tr.tags << tg
tr.save

tg.torrents << tr
tg.save

# destroying relation

TagTorrent.first(:tag => tg, :torrent => tr).destroy!

# or
tr.tag_torrents(:tag => tg).destroy

# or
tg.tag_torrents(:torrent => tr).destroy

# destroy items

tr.destroy!
tg.destroy!