Solve unique validation problem within nested simultaneous destruction and creation

unique_collected_validator.rb

class UniqueCollectedValidator < ActiveRecord::Validations::UniquenessValidator
  def initialize(options)
    unless options[:within].present?
      raise ArgumentError, ":within option is required to determine the unique scope's collection."
    end
    unless Array(options[:within]).all? { |within| within.respond_to?(:to_sym) }
      raise ArgumentError, "#{options[:within]} is not a supported format for the :within option. " \
        "Pass a symbol or an array of symbols instead: `within: :profiles`"
    end
    super
  end

  # Adds an additional constraint on the database query to exclude any siblings
  # marked for destruction.
  def scope_relation(record, relation)
    relation = super

    Array(options[:scope]).each do |scope_item|
      scope_value = record.association(scope_item).reader
      inverse_association = options[:within]
      scope_collection = scope_value.association(inverse_association).reader

      dead_siblings = scope_collection.grep(:marked_for_destruction?.to_proc, &:id_in_database)

      if dead_siblings.present?
        finder_class = find_finder_class_for(record)
        relation = relation.where.not(finder_class.primary_key => dead_siblings)
      end
    end

    relation
  end
end

Example use

class Account < ApplicationRecord
  has_many :profiles
  accepts_nested_attributes_for :profiles

class Profile < ApplicationRecord
  belongs_to :account
  validates :email, unique_collected: { scope: :account, within: :profiles }

Now an update with nested attributes can simultaneously destroy and create a profile with the same email address.

Caveat: depends on Rails internal API. Tested with Rails 5.2 only.