# Möglichkeit 2 Wohl die bessere Lösung mit inverse_of und validates_associated.

gistfile1.rb

class Plan < ActiveRecord::Base

  ...

  has_many :plan_additions, :inverse_of => :plan
  validates_associated :plan_additions

  accepts_nested_attributes_for :plan_additions

  ...

end

class PlanAddition < ActiveRecord::Base

  ...

  belongs_to :plan

  validates :name, :presence => true
  
  validates :monthly_price, presence: { if: -> { plan.monthly_price? } }

  ...

end