Preserve localized slug history across fallbacks

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## 7.1.1 (Next)
 
+* [#184](https://github.com/mongoid/mongoid-slug/issues/184): Preserve default-locale slug history when another locale uses the same title through I18n fallbacks - [@dblock](https://github.com/dblock).
 * [#284](https://github.com/mongoid/mongoid-slug/pull/284): Cover saved slug mutation, suggestions, history-state reporting, require aliases and real Rails task registration; combine supported-version coverage - [@dblock](https://github.com/dblock).
 * Removed Code Climate badge from README - [@dblock](https://github.com/dblock).
 * Your contribution here.

lib/mongoid/slug.rb

@@ -197,9 +197,6 @@ def apply_slug
       # to find document instead
       return true if new_slug.empty?
 
-      # avoid duplicate slugs
-      _slugs&.delete(new_slug)
-
       if !!slug_history && _slugs.is_a?(Array)
         append_slug(new_slug)
       else
@@ -284,10 +281,12 @@ def append_slug(value)
         # This is necessary for the scenario in which the slugged locale is not yet present
         # but the default locale is. In this situation, self._slugs falls back to the default
         # which is undesired
-        current_slugs = _slugs_translations.fetch(I18n.locale.to_s, [])
+        current_slugs = _slugs_translations.fetch(I18n.locale.to_s, []).dup
+        current_slugs.delete(value)
         current_slugs << value
         self._slugs_translations = _slugs_translations.merge(I18n.locale.to_s => current_slugs)
       else
+        _slugs.delete(value)
         _slugs << value
       end
     end

spec/mongoid/localized_fallback_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'i18n/backend/fallbacks'
+
+describe 'Localized slug history with fallbacks' do
+  around do |example|
+    original_fallbacks = I18n.fallbacks
+    I18n.fallbacks = I18n::Locale::Fallbacks.new(nl: :en)
+    I18n.with_locale(:en) { example.run }
+  ensure
+    I18n.fallbacks = original_fallbacks
+  end
+
+  it 'preserves the default locale when adding the same title in another locale' do
+    page = PageSlugLocalizedHistory.create!(title: 'Shared Title')
+
+    I18n.with_locale(:nl) { page.update!(title: 'Shared Title') }
+
+    expect(page.reload._slugs_translations).to eq(
+      'en' => ['shared-title'], 'nl' => ['shared-title']
+    )
+  end
+
+  it 'keeps independent histories without duplicate current slugs' do
+    page = PageSlugLocalizedHistory.create!(title: 'Original Title')
+    I18n.with_locale(:nl) { page.update!(title: 'Original Title') }
+    I18n.with_locale(:nl) { page.update!(title: 'Dutch Title') }
+    I18n.with_locale(:nl) { page.update!(title: 'Original Title') }
+
+    expect(page.reload._slugs_translations).to eq(
+      'en' => ['original-title'], 'nl' => %w[dutch-title original-title]
+    )
+  end
+end