What is the differences when putting autosave: true on belongs_to and has_many?

rubyonrails-talk
1

so I got confused about AutosaveAssociation

so lets say I have this class

class Bill < ApplicationRecord
  has_many :bill_entries, dependent: :destroy
  accepts_nested_attributes_for :bill_entries, reject_if: :all_blank, allow_destroy: true
end

class BillEntry < ApplicationRecord
  belongs_to :bill
  before_validation :set_amount

  def set_amount
    self.amount = self.price * self.quantity * self.bill.currency_rate
  end
end

schema.rb

ActiveRecord::Schema[7.0].define(version: 2022_03_10_061810) do
  create_table "bills", force: :cascade do |t|
    t.bigint "company_id"
    t.decimal "vat", default: "0.0", null: false
    t.decimal "currency_rate", default: "0.0", null: false
  end

  create_table "bill_entries", force: :cascade do |t|
    t.bigint "bill_id", null: false
    t.decimal "quantity"
    t.decimal "price", default: "0.0", null: false
    t.decimal "amount", null: false
    t.index ["bill_id"], name: "index_bill_entries_on_bill_id"
  end
end

why whenever I save or edit the parents the child validation is not called? even though I have add autosave: true to the parent like this

class Bill < ApplicationRecord
  has_many :bill_entries, dependent: :destroy, autosave: true
  accepts_nested_attributes_for :bill_entries, reject_if: :all_blank, allow_destroy: true
end

and when I put the autosave: true to the child like this instead the parent the validation is called

class BillEntry < ApplicationRecord
  belongs_to :bill, autosave: true
  before_validation :set_amount

  def set_amount
    self.amount = self.price * self.quantity * self.bill.currency_rate
  end
end

what is the difference between them? I got confused the doc in options for belongs_to and options for has_many have the same words for them but it gives difference behavior