AO3-4122 Prevent duplicate translation languages for admin posts

app/models/admin_post.rb

@@ -34,6 +34,13 @@ class AdminPost < ApplicationRecord
   validates_length_of :content, maximum: ArchiveConfig.CONTENT_MAX,
     too_long: ts("cannot be more than %{max} characters long.", max: ArchiveConfig.CONTENT_MAX)
 
+  # rubocop:disable Rails/UniqueValidationWithoutIndex
+  validates :translated_post_id,
+            uniqueness: { scope: :language_id,
+                          message: :translation_already_exists },
+            allow_nil: true
+  # rubocop:enable Rails/UniqueValidationWithoutIndex
+
   validate :translated_post_must_exist
 
   validate :translated_post_language_must_differ

config/locales/models/en.yml

@@ -102,6 +102,7 @@ en:
               invalid_permissions: Comment permissions are invalid.
             translated_post_id:
               must_be_posted_first: cannot be posted before the original post
+              translation_already_exists: already has a translation in this language
         block:
           attributes:
             blocked:

features/admins/admin_post_news.feature

@@ -66,6 +66,15 @@ Feature: Admin Actions to Post News
       And I am logged in as "ordinaryuser"
     Then I should see a translated admin post
 
+  Scenario: Make a second translation of an admin post in the same language
+    Given I have posted an admin post
+      And basic languages
+      And the admin post "Erste Ankuendigung" translating "Default Admin Post" to "Deutsch"
+      And I am logged in as a "translation" admin
+    When I make a translation of an admin post
+    Then I should see "Sorry! We couldn't save this admin post because:"
+      And I should see "Translated post already has a translation in this language"
+
   Scenario: Make a translation of an admin post that doesn't exist
     Given basic languages
       And I am logged in as a "translation" admin

features/step_definitions/admin_steps.rb

@@ -160,6 +160,16 @@
   FactoryBot.create(:admin_post, :draft, title: title, comment_permissions: :enable_all, tag_list: tags)
 end
 
+Given "the admin post {string} translating {string} to {string}" do |title, translated_title, lang|
+  admin_post = AdminPost.find_by!(title: translated_title)
+  language = Language.find_by!(name: lang)
+  FactoryBot.create(:admin_post,
+                    title: title,
+                    comment_permissions: :enable_all,
+                    translated_post_id: admin_post.id,
+                    language: language)
+end
+
 Given "the draft admin post {string} translating {string} to {string}" do |title, translated_title, lang|
   admin_post = AdminPost.find_by!(title: translated_title)
   language = Language.find_by!(name: lang)

spec/models/admin_post_spec.rb

@@ -78,6 +78,61 @@
     end
   end
 
+  describe "#translated_post_id" do
+    let(:admin_post) { create(:admin_post) }
+    let(:language) { create(:language) }
+    let(:translation) do
+      build(:admin_post, translated_post: admin_post, language: language)
+    end
+    let(:message) { "already has a translation in this language" }
+
+    context "when the original post has a posted translation in the language" do
+      before do
+        create(:admin_post, translated_post: admin_post, language: language)
+      end
+
+      it "adds an error to the translated post" do
+        expect(translation).to be_invalid
+        expect(translation.errors[:translated_post_id]).to include(message)
+      end
+    end
+
+    context "when the original post has a draft translation in the language" do
+      before do
+        create(:admin_post, :draft,
+               translated_post: admin_post, language: language)
+      end
+
+      it "adds an error to the translated post" do
+        expect(translation).to be_invalid
+        expect(translation.errors[:translated_post_id]).to include(message)
+      end
+    end
+
+    context "when another post has a translation in that language" do
+      before do
+        other_post = create(:admin_post)
+        create(:admin_post, translated_post: other_post, language: language)
+      end
+
+      it "does not add an error" do
+        expect(translation).to be_valid
+      end
+    end
+
+    context "when updating the existing translation" do
+      let!(:existing) do
+        create(:admin_post, translated_post: admin_post, language: language)
+      end
+
+      it "does not add an error" do
+        existing.title = "Updated translation title"
+
+        expect(existing).to be_valid
+      end
+    end
+  end
+
   describe "#set_published_at" do
     before { freeze_time }