Rails - 过滤多对多

Posted 2023-03-22

【中文标题】Rails - 过滤多对多

【英文标题】：Rails - filter many to many

【发布时间】：2021-12-09 23:58:11

【问题描述】：

“事件”和“成员”之间存在多对多关系。我目前正在尝试过滤掉不属于某个成员的所有事件。

我可以做相反的事情，我可以选择属于特定事件的所有事件。我就是这样做的：

class Event < ApplicationRecord
  has_and_belongs_to_many :members
  scope :filter_by_member, -> (member)  joins(:members).where( members:  id: member.id  ) 
end

class Member < ApplicationRecord
  has_and_belongs_to_many :events
end

但是，我试图通过将where 更改为where.not 来获得反向行为（所有不属于成员的事件）；这只是选择事件表中的所有记录。

在将where 切换为where.not 后，我试图在rails 控制台中获取所有不属于成员的事件。但是，正确的输出应该是在最后一行中选择了事件 2 和 3（因为事件 1 属于 sample_member）；三个都被选中了。

irb(main):001:0> sample_member = Member.find(2)
  Member Load (0.3ms)  SELECT "members".* FROM "members" WHERE "members"."id" = $1 LIMIT $2  [["id", 2], ["LIMIT", 1]]
=> #<Member id: 2, uid: nil, name: "John", created_at: "2021-10-23 14:30:38.434442000 -0500", updated_at: "2021-10-23 14:30:38.434442000 -0500", isAdmin: nil, email: nil>
irb(main):002:0> sample_member.events
  Event Load (0.4ms)  SELECT "events".* FROM "events" INNER JOIN "events_members" ON "events"."id" = "events_members"."event_id" WHERE "events_members"."member_id" = $1  [["member_id", 2]]
=> 
[#<Event:0x00007f02f0aeb4e8
  id: 1,
  date: nil,
  description: "Please come",
  isMandatory: nil,
  location: nil,
  admin_id: nil,
  created_at: Sat, 23 Oct 2021 14:27:32.966994000 CDT -05:00,
  updated_at: Sat, 23 Oct 2021 14:27:32.966994000 CDT -05:00,
  time: nil,
  datetime: nil,
  name: "Fun event">]
irb(main):003:0> Event.filter_by_member sample_member
  Event Load (0.3ms)  SELECT "events".* FROM "events" INNER JOIN "events_members" ON "events_members"."event_id" = "events"."id" INNER JOIN "members" ON "members"."id" = "events_members"."member_id" WHERE "members"."id" != $1  [["id", 2]]
=> 
[#<Event:0x00005651319bf3d8
  id: 1,
  date: nil,
  description: "Please come",
  isMandatory: nil,
  location: nil,
  admin_id: nil,
  created_at: Sat, 23 Oct 2021 14:27:32.966994000 CDT -05:00,
  updated_at: Sat, 23 Oct 2021 14:27:32.966994000 CDT -05:00,
  time: nil,
  datetime: nil,
  name: "Fun event">,
 #<Event:0x00005651319bf270
  id: 2,
  date: nil,
  description: "Important",
  isMandatory: nil,
  location: nil,
  admin_id: nil,
  created_at: Sat, 23 Oct 2021 14:29:39.380136000 CDT -05:00,
  updated_at: Sat, 23 Oct 2021 14:29:39.380136000 CDT -05:00,
  time: nil,
  datetime: nil,
  name: "Conference">,
 #<Event:0x00005651319bf158
  id: 3,
  date: nil,
  description: "this is orientation",
  isMandatory: false,
  location: "",
  admin_id: nil,
  created_at: Sat, 23 Oct 2021 14:39:13.186162000 CDT -05:00,
  updated_at: Sat, 23 Oct 2021 14:39:13.186162000 CDT -05:00,
  time: nil,
  datetime: nil,
  name: "orientation">]

有什么线索吗？

【问题讨论】：

你首先不需要那个范围。只需member.events。

【参考方案1】：

最简单的方法是使用WHERE events.id NOT IN (...) 子查询：

Event.where.not(
  id: member.events
)