为什么这些named_scopes会导致重复的INNER JOIN？

Question

我有一个模型，用于使用 awesome_nested_set 插件跟踪分层组织中的权限。我遇到了一个问题，两个 named_scope 链接在一起时会创建重复的 INNER JOIN。

class Group < ActiveRecord::Base
  acts_as_nested_set
  has_many :memberships
  has_many :accounts, :through => :memberships
  has_many :authorizations
  has_many :users, :through => :authorizations
end

这两个 named_scope 位于 Account 模型中，用于按用户和组筛选帐户：

named_scope :in_group, lambda { |id|
  group_ids = Group.find(id).self_and_descendants.collect(&:id)
  { :joins => :memberships, :conditions => ["memberships.group_id in (?)", group_ids] }
}

named_scope :for, lambda { |user|
  groups = user.authorizations.groups.collect(&:id) unless user.admin?
  user.admin ? {} : { :joins => :groups, :conditions => ["groups.id IN (?)", groups] }
}

这两个 named_scope需要加入会员资格，但他们不应该能够在不重复的情况下做到这一点吗？当它们链接在一起时，mysql 会崩溃并出现以下错误：

Mysql::Error: Not unique table/alias: 'memberships': SELECT `accounts`.* FROM `accounts` INNER JOIN `memberships` ON accounts.id = memberships.account_id INNER JOIN `memberships` ON `accounts`.`id` = `memberships`.`account_id` INNER JOIN `groups` ON `groups`.`id` = `memberships`.`group_id` WHERE ((memberships.group_id in (54,94)) AND (groups.id IN (54,94)))  ORDER BY business_name, location_name LIMIT 0, 75

Answer 1

尝试将 for 命名范围中的 join 语句更改为 :joins => {：会员=> ：组}。我怀疑 has_many 通过关系可能会导致它。