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This gem was born out of the need to have an established universal error format to be consumed by frontend (JS), Android and iOS clients.

API error format

In our projects we have a convention, that in case of a failed request (422) backend shall return a JSON response which conforms to the following schema:

  errors: [{
    key: 'has_already_been_taken',
    type: 'params',
    message: 'has already been taken',
    payload: {
      path: ''

Each error object must have 4 required fields: key, type, message and payload.

  • key is a concise error code that will be used in a user-friendly translations
  • type may be params, custom, rule or something else
    • params means that some parameter that the backend received was wrong
    • rule covers cases of the high-level (business) rules which validate against several fields simultaneously
    • custom covers everything else
  • message is a "default" or "fallback" error message in plain English that may be used if client does not have translation for the error code
  • payload contains other useful data that assists client error handling. For example, in case of type: "params" we can provide a path to the invalid paramter.




GIVEN following dry-validation schema

schema = Dry::Validation.Schema do
  required(:name).filled(size?: 3..15)
  required(:email).filled(format?: /\A([\w+\-].?)[email protected][a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z/i)

  rule(payment: [:credit_card, :cash]) do |card, cash|
    card.eql?(true) ^ cash.eql?(true)

AND following input

errors = 'DK', email: 'dk<@>').errors
# {:name=>["length must be within 3 - 15"], :credit_card=>["must be filled"]}

THEN we can convert given errors to API error format

# [{
#   :key=>"length_must_be_within",
#   :message=>"length must be within 3 - 15",
#   :payload=>{:path=>"name", :range=>["3", "15"]},
#   :type=>"params"
# }, {
#   :key=>"is_in_invalid_format",
#   :message=>"is in invalid format",
#   :payload=>{:path=>"email"},
#   :type=>"params"
# }, {
#   :key=>"must_be_equal_to",
#   :message=>"must be equal to true",
#   :payload=>{:path=>"payment", :value=>"true"},
#   :type=>"params"
# }]

For more information about supported errors and how they would be parsed please check the spec.

Type-inference feature

TL;DR: Add _rule to the custom validation block names (adding this to the high-level rules won't harm either, praise the consistency!).

Long version: When you're using custom validation blocks the error output is slightly diffenet. Instead of the attribute name it will have a rule name as a key. For example, GIVEN this schema

schema = Dry::Validation.Schema do
  configure do
    def self.messages
      super.merge(en: { errors: { email_required: 'provide email' } })


  validate(email_required: %i[newsletter email]) do |newsletter, email|
    if newsletter == true

AND the following input

errors = true, email: nil).errors
# { email_required: ['provide email'] }

THEN we will get following format after normalization

# [{
#   key: 'provide_email',
#   message: 'provide email',
#   payload: { path: 'email_required' }, # should be empty to not confuse ppl
#   type: 'params' # should be "rule" or "custom" but definately not "params"
# }]

The solution to this problem would be to use type inference from the rule name feature. Just add a _rule to the name of a custom block validation, like this

validate(email_required_rule: %i[newsletter email]) do |newsletter, email|

Now validation will produce errors like this

{ email_required_rule: ['provide email'] }

But we can easily spot keys which end with _rule and normalize such erros appropiately to the following format

ErrorNormalizer.normalize(email_required_rule: ['provide email'])
# [{
#   key: 'provide_email',
#   message: 'provide email',
#   payload: {},
#   type: 'rule'
# }]

You can customize rule name match pattern, type name or turn off this feature completely by specifying it in configuration block

ErrorNormalizer.configure do |config|
  config.infer_type_from_rule_name = true
  config.rule_matcher = /_rule\z/
  config.type_name = 'rule'

I18n support

Full message translation

This feature allows to define the localization for schema attributes (think of path that you get in payload), translate it with I18n and concatenate it with the error messages.

schema = Dry::Validation.Schema do
  required(:user).schema do
    required(:favorite_pet).filled(size?: 3..8)
    required(:vessel).schema do
      required(:factory).filled(excluded_from?: ['Bilgewater', 'Shipwreck'])

AND following input

errors = schema.(user: { favorite_pet: 'Zuckerberg', vessel: { factory: 'Bilgewater' } }).errors
#=> {:user=>{:favorite_pet=>["length must be within 3 - 8"], :company=>{:name=>["must not be one of: Bilgewater, Shipwreck"]}}}

AND following translations loaded in I18n

      '@': cap
      favorite_pet: parrot
        '@': ship
        factory: dockyard

THEN we can convert it to fully translated errors

ErrorNormalizer.normalize(errors, i18n_messages: true)
# [{
#   :key=>"length_must_be_within",
#   :message=>"Cap parrot length must be within 3 - 8",
#   :payload=>{:path=>"user.favorite_pet", :range=>["3", "15"]},
#   :type=>"params"
# }, {
#   :key=>"must_not_be_one_of",
#   :message=>"Cap ship dockyard must not be one of: Bilgewater, Shipwreck",
#   :payload=>{:path=>"user.vessel.factory", :list=>["Bilgewater", "Shipwreck"]},
#   :type=>"params"
# }]

You can configure this behaviour globally:

ErrorNormalizer.configure do |config|
  config.i18n_messages = true

For the i18n lookup rules go check SchemaPathTranslator documentation.

Non-english error messages

If you want to support error messages for the other languages you'll need to define and register localized message parser. You can register it in configuration block:

ErrorNormalizer.configure do |config|
  config.message_parsers << RussianMessageParser

For message parser implementation please check the documentation and the source code of ErrorNormalizer::MessageParser::English.


ActiveModel errors aren't fully supported. By that I mean errors will be converted to the single format, however you won't see really unique error key or payload with additional info.

GIVEN we have a model like this

class TestUser
  include ActiveModel::Validations

  attr_reader :name, :email
  def initialize(name:, email:)
    @name = name
    @email = email

  validates :name, presence: true, length: { in: 3..15 }
  validates :email, presence: true, format: { with: /\A([\w+\-].?)[email protected][a-z\d\-]+(\.[a-z]+)*\.[a-z]+\z/i }

AND initialzied object with invalid data

user = 'DK', email: 'dk<@>').tap(&:validate)

THEN we can normalize object errors to API error format

# [{
#   :key=>"is_too_short_minimum_is_3_characters",
#   :message=>"is too short (minimum is 3 characters)",
#   :payload=>{:path=>"name"},
#   :type=>"params"
# }, {
#   :key=>"is_invalid",
#   :message=>"is invalid",
#   :payload=>{:path=>"email"},
#   :type=>"params"
# }]


The gem is available as open source under the terms of the MIT License.