Scope with join on :has_many :through association

问题:

class Users < ActiveRecord::Base
  has_many :meetings, :through => :meeting_participations
  has_many :meeting_participations
end

class Meetings < ActiveRecord::Base
  has_many :users, :through => :meeting_participations
  has_many :meeting_participations
end

class MeetingParticipations < ActiveRecord::Base
  belongs_to :user
  belongs_to :meeting

  scope :hidden, where(:hidden => true)
  scope :visible, where(:hidden => false)
end

hidden is an extra boolean column within the m2m association table. Given some Users instance current_user, I want to do

current_user.meetings.visible

which will retrieve a collection of Meetings for which the user is a participant where the hidden column is false. The closest I have gotten is adding the following scope to the Meetings class

scope :visible, joins(:meeting_participations) & MeetingParticipation.visible

The scope does filter the Meetings against the MeetingParticipations table, however there is no join/condition against the MeetingParticipations table related to current_user.

The issue with this is, if current_user and another_user are both participants for some Meetings instance, a Meetings record in the result set will be returned for each participant that has hidden set to false. If current_user has true set for hidden for all Meetings, if another_user is a participant in any of those same Meetings with hidden set to false, those Meetings will appear in the Meetings.visible result set.

Is it possible to have a scope as I've mentioned above which will properly join on the User instance? If not, can someone recommend a solution to this?

回答1:

This is my solution for your problem:

class User < ActiveRecord::Base
  has_many :meeting_participations
  has_many :meetings, :through => :meeting_participations do
   def visible
     where("meeting_participations.visible = ?", true)
   end
  end
end

@user.meetings.visible

回答2:

In Rails 4, you can specify the scope originally defined in the child object in the association itself. Short: you don't have to know the internals of the MeetingParticipation model within the User model.

class User < ActiveRecord::Base
  has_many :meeting_participations
  has_many :meetings, :through => :meeting_participations
  has_many :visible_participations, -> { visible }, :class_name => 'MeetingParticipation'
  has_many :visible_meetings, :source => :meeting, :through => :visible_participations
end

class Meeting < ActiveRecord::Base
  has_many :meeting_participations
  has_many :users, :through => :meeting_participations
end

class MeetingParticipation < ActiveRecord::Base
  belongs_to :user
  belongs_to :meeting

  scope :hidden, -> { where(:hidden => true) }
  scope :visible, -> { where(:hidden => false) }
end

This would allow you to do: user1.visible_meetings and user2.visible_meetings with different result sets

回答3:

current_user.meetings.merge(MeetingParticipations.visible)

回答4:

Here's a one liner:

Meeting.joins(:meeting_participations).where(meeting_participation: { hidden: false, user_id: current_user.id })

This is great because you can make a scope out of it, a function out of it, or simply call it anywhere. You can also add any more restrictions you want to the hash.

回答5:

It would seem to me that it is not sensible to use a scope on Meeting for your purpose. A meeting itself has no visibility, but the participation has. So I would suggest an extension on the association within User:

class User < ActiveRecord::Base
  has_many :meetings, :through => :meeting_participations do
    def visible
      ids = MeetingParticipation.
        select(:meeting_id).
        where(:user_id => proxy_owner.id, :visible => true).
        map{|p| p.meeting_id}
      proxy_target.where("id IN (?)", ids)
    end
  end
  ...
end

I hope, this helps.

回答6:

You could also do:

current_user.meeting_participations.visible.map(&:meeting)