Rails 5 - Problem with Umlaute and parameterize method - ruby-on-rails

Question

I work on Rails 5.2.3 and Ruby 2.5.1. At some point, I found an issue when I expected my array-of-string constant to contain some string but it didn't. Turned out the problem was related to German Umlaute characters (öäü).
So I have the constant defined like the following:
# coding: utf-8
# frozen_string_literal: true
class MyClass
module MyModule
MY_CONSTANT = [
'Breite in mm',
'Höhe in mm',
'Länge in mm'
].map(&:parameterize).freeze
end
end
I expect the constant to look like ["breite-in-mm", "hoehe-in-mm", "laenge-in-mm"]
But instead it's stored as ["breite-in-mm", "hohe-in-mm", "lange-in-mm"]. You see, "ö" has been converted to "o" instead of "oe". Same for "ä". Now it's "a", not "ae".
It works this way on production, in RSpec tests and even when I start Rails console and call this constant. But when I define a new constant from Rails console using the very same code, the strings are being successfully converted to what I expect, i.e. ["breite-in-mm", "hoehe-in-mm", "laenge-in-mm"]
I could easily get rid of this parameterize method and just type in the strings as I need them. Maybe I will have to do that. But I'm really curious about why all this is happening and couldn't find an answer by myself.
So thank you in advance for any ideas.

Answer 1

The parameterize method in Rails (through its use of ActiveSupport::Inflector#transliterate) is in locale aware. It thus uses locale-depending rules to transliterate characters such as Umlauts to ASCII characters.
When your app handles a request (or at least once after booting), you are usually setting a I18n locale, e.g. with I18n.locale = :de for a single request or with I18n.default_locale = :de for your whole app. After that, Rails (resp. the i18n gem) used this locale by default for its transliteration rules.
When initially setting your constant, this default locale was likely not yet set. The i18n gem is thus not aware of the German transliteration rules and uses only the basic Unicode normalization rules.
As a workaround, you can either pass the desired locale to use to the parameterize method as
MY_CONSTANT = [
'Breite in mm',
'Höhe in mm',
'Länge in mm'
].map { |const| const.parameterize(locale: :de).freeze }.freeze
or you can alternatively set the default i18n locale earlier than when your code is executed (e.g. in a file in config/initializers, depending on where exactly you initialize your constant):
I18n.default_locale = :de

Answer 2

Thanks, Holger Just for your great answer. It seems to be correct except it only works for Rails 6.0.0. So I'm going to post the one for Rails 5.2.3 which I'm using on my project.
Unfortunately in Rails 5 parameterize method does not accept the locale argument yet. This will be possible only in Rails 6.
But still, as mentioned in Holger Just's answer, parameterize method relies on transliterate method which does actually use current locale and converts strings according to it.
See Rails 5.2.3 docs and sources for those methods:
https://api.rubyonrails.org/v5.2.3/classes/ActiveSupport/Inflector.html#method-i-parameterize
https://api.rubyonrails.org/v5.2.3/classes/ActiveSupport/Inflector.html#method-i-transliterate
So I cannot pass the locale to parameterize method directly. Then I should set the locale before my constant is defined.
Setting I18n.default_locale = :de inside application.rb file did not help. I already had that and the strings have been transliterated regardless.
What eventually helped was setting I18n.locale = :de manually. Thanks to this, I got my strings parameterized correctly without any changes to MyConstant definition.