A simple yet effective GraphQL error builder for Ruby

error_objects_builder.rb

# frozen_string_literal: true

# BasicService implements `self.call(*args)` so that you can 
# use `ErrorObjectsBuilder.call(errors)` instead of 
# `ErrorObjectsBuilder.new(errors).call`
class ErrorObjectsBuilder < BasicService
  Error = Struct.new(:key, :index, :messages, :suberrors)

  def initialize(errors, camelize_keys = true)
    @errors = errors
    @camelize_keys = camelize_keys
  end

  def call
    return [] unless errors
    errors.map { |key, messages| build_error(key, messages) }
  end

  protected

  attr_reader :errors
  attr_reader :camelize_keys

  private

  def build_error(key, messages)
    index = nil
    suberrors = nil

    if messages.is_a?(Hash)
      suberrors =
        messages.map { |subkey, submessages| build_error(subkey, submessages) }
      messages = nil
    end

    if key.class == Integer
      index = key.dup
      key = nil
    else
      key = camelize_key(key)
    end

    Error.new(key, index, messages && Array(messages), suberrors)
  end

  def camelize_key(key)
    return key unless camelize_keys
    return key unless key.respond_to?(:to_s)
    key.to_s.camelize(:lower)
  end
end

# The output of this is an array of hashes that has the following structure
# ```JSON
# {
#   "key": "password",
#   "messages": [
#.    "must be longer than 8 characters", 
#     "must contain a symbol"
#   ],
#   "index": null,
#   "suberrors": null
# }
# ```
#
# If there is a nested error. Say, that an user is creating 
# a gallery of images and the 5th image is actually a video
# (has an invalid file type).
# ```JSON
# {
#   "key": "images",
#   "messages": null,
#   "index": null,
#   "suberrors": [
#     {
#       "key": "password",
#       "messages": [
#.        "must be an image (jpeg/gif/png/bmp)"
#       ],
#       "index": 4,
#       "suberrors": null
#     }
#.  ]
# }
# ```
# Conversly, the same pattern can be applied for nested
# objects (then the index field is `null`).
#
# In later versions the first element of the `messages` array
# has been exposed under the key `message` though this is not
# necessary. It's here to add compatibility with standard GQL
# errors and therefor with existing libraries. E.g.
# ```JSON
# {
#   "key": "password",
#.  "message": "Password must be longer than 8 characters",
#   "messages": [
#.    "must be longer than 8 characters", 
#     "must contain a symbol"
#   ],
#   "index": null,
#   "suberrors": null
# }
# ```