Fix crash when available locales are enforced but fallback locale unavailable

CHANGELOG.md

@@ -9,6 +9,7 @@
 #### Fixes
 
 * Your contribution here.
+* [#1796](https://github.com/ruby-grape/grape/pull/1796): Fix crash when available locales are enforced but fallback locale unavailable - [@Morred](https://github.com/Morred).
 * [#1776](https://github.com/ruby-grape/grape/pull/1776): Validate response returned by the exception handler - [@darren987469](https://github.com/darren987469).
 * [#1787](https://github.com/ruby-grape/grape/pull/1787): Add documented but not implemented ability to `.insert` a middleware in the stack - [@michaellennox](https://github.com/michaellennox).
 * [#1788](https://github.com/ruby-grape/grape/pull/1788): Fix route requirements bug - [@darren987469](https://github.com/darren987469), [@darrellnash](https://github.com/darrellnash).

README.md

@@ -1559,6 +1559,8 @@ end
 Grape supports I18n for parameter-related error messages, but will fallback to English if
 translations for the default locale have not been provided. See [en.yml](lib/grape/locale/en.yml) for message keys.
 
+In case your app enforces available locales only and :en is not included in your available locales, Grape cannot fall back to English and will return the translation key for the error message. To avoid this behaviour, either provide a translation for your default locale or add :en to your available locales.
+
 ### Custom Validation messages
 
 Grape supports custom validation messages for parameter-related and coerce-related error messages.

lib/grape/exceptions/base.rb

@@ -74,7 +74,15 @@ def translate(key, **options)
         options = options.dup
         options[:default] &&= options[:default].to_s
         message = ::I18n.translate(key, **options)
-        message.present? ? message : ::I18n.translate(key, locale: FALLBACK_LOCALE, **options)
+        message.present? ? message : fallback_message(key, **options)
+      end
+
+      def fallback_message(key, **options)
+        if ::I18n.enforce_available_locales && !::I18n.available_locales.include?(FALLBACK_LOCALE)
+          key
+        else
+          ::I18n.translate(key, locale: FALLBACK_LOCALE, **options)
+        end
       end
     end
   end

spec/grape/exceptions/base_spec.rb

@@ -0,0 +1,61 @@
+require 'spec_helper'
+
+describe Grape::Exceptions::Base do
+  describe '#compose_message' do
+    subject { described_class.new.send(:compose_message, key, attributes) }
+
+    let(:key) { :invalid_formatter }
+    let(:attributes) { { klass: String, to_format: 'xml' } }
+
+    after do
+      I18n.available_locales = %i[en]
+      I18n.default_locale = :en
+    end
+
+    context 'when I18n enforces available locales' do
+      before { I18n.enforce_available_locales = true }
+
+      context 'when the fallback locale is available' do
+        before do
+          I18n.available_locales = %i[de en]
+          I18n.default_locale = :de
+        end
+
+        it 'returns the translated message' do
+          expect(subject).to eq('cannot convert String to xml')
+        end
+      end
+
+      context 'when the fallback locale is not available' do
+        before do
+          I18n.available_locales = %i[de jp]
+          I18n.default_locale = :de
+        end
+
+        it 'returns the translation string' do
+          expect(subject).to eq("grape.errors.messages.#{key}")
+        end
+      end
+    end
+
+    context 'when I18n does not enforce available locales' do
+      before { I18n.enforce_available_locales = false }
+
+      context 'when the fallback locale is available' do
+        before { I18n.available_locales = %i[de en] }
+
+        it 'returns the translated message' do
+          expect(subject).to eq('cannot convert String to xml')
+        end
+      end
+
+      context 'when the fallback locale is not available' do
+        before { I18n.available_locales = %i[de jp] }
+
+        it 'returns the translated message' do
+          expect(subject).to eq('cannot convert String to xml')
+        end
+      end
+    end
+  end
+end

spec/spec_helper.rb

@@ -12,7 +12,9 @@
   require file
 end
 
-I18n.enforce_available_locales = false
+# The default value for this setting is true in a standard Rails app,
+# so it should be set to true here as well to reflect that.
+I18n.enforce_available_locales = true
 
 RSpec.configure do |config|
   config.include Rack::Test::Methods