class ActiveRecord::Validations::UniquenessValidator

def resolve_attributes(record, attributes)
  attributes.flat_map do |attribute|
    reflection = record.class._reflect_on_association(attribute)
    if reflection.nil?
      attribute.to_s
    elsif reflection.polymorphic?
      [reflection.foreign_key, reflection.foreign_type]
    else
      reflection.foreign_key
    end
  end
end