An RSpec matcher for doing precise checks on active model validations errors.

have_validation_errors.rb

##
# An RSpec matcher for checking Rails validation errors on an object.
#
# Usage:
#   # Expect exactly three validation errors:
#   #  - field1 must not be empty
#   #  - field1 must be a number
#   #  - field2 must be greater than zero
#   expect(x).to have_validation_errors
#     .related_to(:field1)
#     .that_say("must not be empty", "must be a number")
#     .and_others
#     .related_to(:field2)
#     .that_say("must be greater than zero")
#
#   # Expect no validation errors
#   expect(x).not_to have_validation_errors
#
#   # Expect no validation errors on field1 that say
#   # "field1 must not be empty" or "field1 must be a number"
#   expect(x).not_to have_validation_errors
#     .related_to(:field1)
#     .that_say("must not be empty", "must be a number")
#
RSpec::Matchers.define :have_validation_errors do
  match do |actual|
    total_error_count = self.total_error_count

    if actual.respond_to?(:errors)
      if total_error_count > 0
        (actual.errors.size == total_error_count) && all_errors_match?(actual)
      else
        actual.errors.size > 0
      end
    end
  end

  match_when_negated do |actual|
    total_error_count = self.total_error_count

    !actual.respond_to?(:errors) ||
      ((total_error_count > 0) && all_errors_excluded?(actual)) ||
      (actual.errors.size == 0)
  end

  failure_message do |actual|
    super_message = super()

    if actual.respond_to?(:errors) && actual.errors
      "#{super_message}, but got #{actual.errors.full_messages}"

    else
      "#{super_message}, but got no validation errors."
    end
  end

  failure_message_when_negated do |actual|
    super_message = super()

    "#{super_message}, but got #{actual.errors.full_messages}"
  end

  chain :related_to do |field|
    @field_errors        ||= {}
    @field_errors[field] ||= []

    @last_field = field
  end

  chain :that_say do |*messages|
    unless @last_field
      raise 'last_field must be preceded by related_to.'
    end

    @field_errors[@last_field] += messages
  end

  chain :and_others do
    # This is a placeholder for fluency
  end

  ##
  # Calculates the total number of expected validation errors across all of the
  # fields.
  #
  # @return [Integer]
  #
  def total_error_count
    if @field_errors
      @field_errors.inject(0) { |memo, (field, errors)| memo + errors.size }
    else
      0
    end
  end

  ##
  # Matches the errors on the provided actual object against this matcher's
  # expectations.
  #
  # @param [ActiveModel::Validations] actual
  #   The object whose validation errors will be matched against this object.
  #
  def all_errors_match?(actual)
    @field_errors.all? do |(field_name, expected_errors)|
      actual_errors = actual.errors[field_name]

      if actual_errors
        expected_errors.all? do |expected_error|
          actual_errors.include? expected_error
        end
      end
    end
  end

  ##
  # Ensures the errors on the provided actual object exclude all of the ones
  # in this matcher's expectations.
  #
  # @param [ActiveModel::Validations] actual
  #   The object whose validation errors will be matched against this object.
  #
  def all_errors_excluded?(actual)
    @field_errors.all? do |(field_name, excluded_errors)|
      actual_errors = actual.errors[field_name]

      actual_errors.blank? || excluded_errors.none? do |excluded_error|
        actual_errors.include? excluded_error
      end
    end
  end
end