class ActiveRecord::Associations::NestedError

def compute_attribute(inner_error)
  association_name = association.reflection.name
  if association.collection? && index_errors_setting && index
    "#{association_name}[#{index}].#{inner_error.attribute}".to_sym
  else
    "#{association_name}.#{inner_error.attribute}".to_sym
  end
end

def index
  @index ||= ordered_records&.find_index(inner_error.base)
end

def index_errors_setting
  @index_errors_setting ||=
    association.options.fetch(:index_errors, ActiveRecord.index_nested_attribute_errors)
end

def initialize(association, inner_error)
  @base = association.owner
  @association = association
  @inner_error = inner_error
  super(@base, inner_error, { attribute: compute_attribute(inner_error) })
end

def ordered_records
  case index_errors_setting
  when true # default is association order
    association.target
  when :nested_attributes_order
    association.nested_attributes_target
  end
end