ActiveModel::Errors are widely used in ActiveRecord and therefore Rails, and while custom messages can be provided to each validation using the message key, it’s much more flexible to instead pass a symbol as the message, which will cause the message to be looked up using the internationalisation framework provided by the i18n gem that is a dependency of Rails. Here are some examples:

validates :title, presence: true, message: "needs to be filled in"
validates :title, presence: true # Will use built-in validation message look up
validates :title, presence: true, message: :blank # ^ The same effect as the above
validate do
  errors.add(:title, :contains_today_i_learned) unless title.includes?("TIL")

Using an internationalised message is great for a number of reasons:

  1. It keeps strings out of your model
  2. It allows for custom error messages to be constrained to a single location
  3. (The obvious) - it allows error messages to be customized based on the current locale

Once these custom errors are built into your model, you may want to add a test to assert that the expected error messages are generated. You can assert against the string itself of course, however if you run your tests in multiple locales, or just want to keep validation message strings out of your model TEST as well as your model, the following method can come in handy.

When you run validations, an instance of ActiveModel::Errors is available as the return value of the errors method on your model instance. This instance is normally used for accessing all the validation errors on a model, but it also has a number of convenience functions - in particular, the one we’re interested in is called generate_message. This method accepts an attribute, and the custom error (the symbol passed to the message option of your validation, or a built-in validation such as presence, inclusion, etc., and returns the string form of the message. It will respect the current setting of I18n.locale when looking up error strings.

Here are some examples of looking up errors on a model (the instance of which is called subject in this example):

subject.errors.generate_message(:title, :blank) 
# => "can't be blank"

subject.errors.generate_message(:title, :contains_today_i_learned)
# => "must include the category 'TIL'"

I18n.locale = "mi-NZ"
subject.errors.generate_message(:title, :contains_today_i_learned)
# => "me whakauru i te tuhinga 'TIL'"

For more information on the I18n translation format, and exactly how these above messages are resolved, see the I18n Rails Guide section on error message scopes.