Rails Self Referencing Attribute inheritance

I have a model called Option which has an self-referential association. An option can have many suboptions and a suboption can have a parent option. I am also using nested attributes by cocoon gem to create multiple suboptions on a form_for option. I can dynamically create suboptions when creating an option on the form.

views/options/_form.html.erb:

<%= form_for @option do |f| %>

  <p>
    <%= f.label :name %><br>
    <%= f.text_field :name %><br>
    <%= f.label :activity %><br>
    <%= f.select :activity_id, options_for_select(activity_array, @option.activity_id)%><br>
  </p>

  <div>
    <div id="suboptions">
      <%= f.fields_for :suboptions do |suboption| %>
        <%= render 'suboption_fields', f: suboption %>
      <% end %>

      <div class="links">
        <%= link_to_add_association 'add suboption', f, :suboptions %>
      </div>
    </div>
  </div>

  <p>
    <%= f.submit "Send" %>
  </p>
<% end %>

model/option.rb:

class Option < ApplicationRecord
  belongs_to :activity
  has_many :option_students
  has_many :students, through: :option_students
  has_many :suboptions, class_name: "Option", foreign_key: "option_id"
  belongs_to :parent, class_name: "Option", optional: true, foreign_key: "option_id"
  accepts_nested_attributes_for :suboptions, allow_destroy: true,
    reject_if: ->(attrs) { attrs['name'].blank? }

  validates :name, presence: true

  after_initialize :set_defaults
  before_update :set_defaults


  def set_defaults
      self.suboptions.each do |sbp|
        sbp.activity_id = self.activity_id
      end
  end

end

Params:

def option_params
    params.require(:option).permit(:name, :activity_id, :students_ids => [], suboptions_attributes: [:id, :name, activity_id, :_destroy])
  end

I would like that each suboption inherit the activity_id attribute from the parent, on creating and on updating. I tried this way by using the set_defaults method on the model and it works for a new option with new nested suboptions, and it also updates the activity_id of suboptions if I update the activity_id of the parent. But if I create another suboption when updating, it does not pass the attribute from parent to the new suboption.

You can use before_validation call back. For example,

Test Code

class Location < ApplicationRecord
  has_many :children, class_name: 'Location', foreign_key: 'parent_id'
  accepts_nested_attributes_for :children
  before_validation :initialize_children

  attr_accessor :activity

  def initialize_children
    children.each { |c| c.activity_id = self.activity_id }
  end
end

Rails Console

irb(main):002:0> Location.create({name: "L10", activity_id: 200, :children_attributes => [{name: "L12"}]})
   (0.1ms)  begin transaction
  SQL (1.1ms)  INSERT INTO "locations" ("name", "activity_id", "created_at", "updated_at") VALUES (?, ?, ?, ?)  [["name", "L10"], ["activity_id", 200], ["created_at", 2017-01-11 04:07:26 UTC], ["updated_at", 2017-01-11 04:07:26 UTC]]
  SQL (0.1ms)  INSERT INTO "locations" ("name", "parent_id", "activity_id", "created_at", "updated_at") VALUES (?, ?, ?, ?, ?)  [["name", "L12"], ["parent_id", "1"], ["activity_id", 200], ["created_at", 2017-01-11 04:07:26 UTC], ["updated_at", 2017-01-11 04:07:26 UTC]]
   (3.7ms)  commit transaction
=> #<Location id: 1, name: "L10", parent_id: nil, activity_id: 200, created_at: "2017-01-11 04:07:26", updated_at: "2017-01-11 04:07:26">
irb(main):003:0> Location.all
  Location Load (0.2ms)  SELECT "locations".* FROM "locations"
=> #<ActiveRecord::Relation [#<Location id: 1, name: "L10", parent_id: nil, activity_id: 200, created_at: "2017-01-11 04:07:26", updated_at: "2017-01-11 04:07:26">, #<Location id: 2, name: "L12", parent_id: "1", activity_id: 200, created_at: "2017-01-11 04:07:26", updated_at: "2017-01-11 04:07:26">]>
irb(main):004:0> Location.last
  Location Load (0.2ms)  SELECT  "locations".* FROM "locations" ORDER BY "locations"."id" DESC LIMIT ?  [["LIMIT", 1]]
=> #<Location id: 2, name: "L12", parent_id: "1", activity_id: 200, created_at: "2017-01-11 04:07:26", updated_at: "2017-01-11 04:07:26">