Topic: accepts_nested_attributes_for using the linking table

I see everywhere that accepts_nested_attributes_for is oriented to nest normal related tables, but what about that third table in a has_many :through association?

class Member < ActiveRecord::Base
  has_many :subscriptions, dependent: :delete_all
  has_many :groups, through: :subscriptions

  attr_accessible :email

  validates :email, presence: true
end

class Group < ActiveRecord::Base
  has_many :subscriptions, dependent: :delete_all
  has_many :members, through: :subscriptions

  accepts_nested_attributes_for :subscriptions

  attr_accessible :name, :subscriptions_attributes

  validates :name, presence: true, uniqueness: true
end

class Subscription < ActiveRecord::Base
  belongs_to :group
  belongs_to :member

  attr_accessible :group_id, :introduction

  validates :group_id, presence: true
  validates :introduction, presence: true
end

I need to create a new group and a subscription for the current member at the same time, so I need to nest the introduction attribute inside the group form. I think that is not possible in Rails (no one seems to know about it, I already asked in the official group), am I wrong or should this be supported?

Here I have a sample app that demonstrates the issue: https://github.com/idavemm/nested_form