Customize i18n of authentication keys like 'email' in auth failure messages

You may notice that by default your auth failure message look something like "Invalid Email or password."

Why is "Email" capitalized? Because Devise is using User.human_attribute_name(:email), which tends to be capitalized.

Note that this issue might be solved (at least for English, as e.g. in German the capitalization is correct) in future versions of devise, with https://github.com/plataformatec/devise/pull/4834

You can control i18n of parameters to authentication failure messages by doing something like this:

In app/models/custom_devise_failure_app.rb:

# Overriding the i18n_options method in the Devise FailureApp so that we can have control
# over how the `authentication_keys` parameter to the messages under devise.failure is translated.
# By default the `human_attribute_name` of the attribute is used, but those are typically capitalized,
# while the messages call for a lowercase value.
# This custom failure app must be activated in the config/initializers/devise.rb file.
class CustomDeviseFailureApp < Devise::FailureApp
  def i18n_options(options)
    options.merge(authentication_keys: I18n.t("devise.authentication_keys.email"))
  end
end

and then in config/initializers/devise.rb:

  config.warden do |manager|
    manager.failure_app = CustomDeviseFailureApp
  end

and then in your devise.yml file:

  devise:
    ...
    authentication_keys:
      email: "email"

Ideally Devise would support an optional authentication_keys key in the YML file out of the box, so you could have control over these params without a custom failure app, but this works right now and is not particularly hacky. There is support for this method in the test suite even, see here.