require 'mongoid'

Mongoid.configure do |c|
  c.clients.default = { hosts: ['localhost:27017'], database: 'mongoid_assoc_validator_repro' }
end

puts "Mongoid version:  #{Mongoid::VERSION}"

class Parent
  include Mongoid::Document
  has_many :children, validate: true
end

class Child
  include Mongoid::Document
  field :note, type: String
  belongs_to :parent

  before_validation :touch_note

  def touch_note
    self.note = "validated at #{Time.now.to_f}"  # writes into the attributes hash
  end
end

Parent.delete_all
Child.delete_all

parent = Parent.create!
child  = parent.children.create!   # created through the association, so it's held in the loaded proxy

# A child is destroyed. In V9, `Persistable::Deletable#prepare_delete` `freeze`s
# the document's attributes hash before flipping `destroyed = true` - and the
# destroyed child is still sitting inside the parent's loaded children proxy.
child.destroy

puts ""
puts "ruby-side:    child.destroyed?          = #{child.destroyed?}"
puts "ruby-side:    child.attributes.frozen?  = #{child.attributes.frozen?}"
puts "ruby-side:    still in parent's proxy?   = #{parent.children.any? { |c| c.equal?(child) }}"

# Validating (or saving) the parent runs `validates_associated :children`, which
# (V9, lazily) walks the in-memory proxy and calls `.valid?` on each element -
# including the destroyed, frozen child. Its before_validation then tries to
# write `note`, mutating the frozen attributes hash.
begin
  parent.valid?
  puts "result:       parent.valid? raised      = (nothing)"
rescue => e
  puts "result:       parent.valid? raised      = #{e.class}: #{e.message}"
end

# Expected:
#   Validating a destroyed document is never useful (it can't be persisted
#   again), so AssociatedValidator should skip it; parent.valid? returns true.
# Actual on mongoid master / 9.1.0:
#   FrozenError: can't modify frozen Hash / BSON::Document
#   because AssociatedValidator calls `.valid?` on the frozen destroyed child,
#   and its mutating before_validation callback writes to the frozen attributes.
#
# Fix: AssociatedValidator#validate_association should skip destroyed documents.
# Its per-value guard is currently
#   value && !value.flagged_for_destroy? && (!value.persisted? || value.changed?)
# Adding `!value.destroyed?` resolves it (a destroyed doc reports
# persisted? == false and flagged_for_destroy? == false, so it slips through).

Mongoid version:  9.1.0

ruby-side:    child.destroyed?          = true
ruby-side:    child.attributes.frozen?  = true
ruby-side:    still in parent's proxy?   = true
result:       parent.valid? raised      = FrozenError: can't modify frozen Hash: {"_id" => BSON::ObjectId('6a1685d9ff90ba5ca8698b14'), "parent_id" => BSON::ObjectId('6a1685d9ff90ba5ca8698b13'), "note" => "validated at 1779860953.9466271"}