Return error when the related content already exists

It looks like this bug was introduced in commit 7ca55c4. Before that
commit, a message saying "You added a new related content" was shown,
but (as expected) the related content wasn't added twice.

We now check this case and add a slightly clearer message.

app/controllers/related_contents_controller.rb

@@ -15,6 +15,8 @@ def create
       flash[:success] = t("related_content.success")
     elsif related_content.same_parent_and_child?
       flash[:error] = t("related_content.error_itself")
+    elsif related_content.duplicate?
+      flash[:error] = t("related_content.error_duplicate")
     else
       flash[:error] = t("related_content.error", url: Setting["url"])
     end

app/models/related_content.rb

@@ -35,6 +35,10 @@ def same_parent_and_child?
     parent_relationable == child_relationable
   end
 
+  def duplicate?
+    parent_relationable.relationed_contents.include?(child_relationable)
+  end
+
   private
 
     def different_parent_and_child

config/locales/en/general.yml

@@ -927,6 +927,7 @@ en:
     help: "You can add links of %{models} inside of %{org}."
     submit: "Add"
     error: "Link not valid. Remember to start with %{url}."
+    error_duplicate: "The related content you are adding already exists."
     error_itself: "Link not valid. You cannot relate a content to itself."
     success: "You added a new related content"
     is_related: "Is it related content?"

config/locales/es/general.yml

@@ -927,6 +927,7 @@ es:
     help: "Puedes introducir cualquier enlace de %{models} que esté dentro de %{org}."
     submit: "Añadir"
     error: "Enlace no válido. Recuerda que debe empezar por %{url}."
+    error_duplicate: "El contenido relacionado que estás añadiendo ya existe."
     error_itself: "Enlace no válido. No se puede relacionar un contenido consigo mismo."
     success: "Has añadido un nuevo contenido relacionado"
     is_related: "¿Es contenido relacionado?"

spec/shared/system/relationable.rb

@@ -117,6 +117,21 @@
     end
   end
 
+  scenario "returns an error when the related content already exists" do
+    create(:related_content, parent_relationable: relationable, child_relationable: related1)
+    login_as(user)
+    visit relationable.url
+
+    click_button "Add related content"
+
+    within("#related_content") do
+      fill_in "url", with: url + related1.url.to_s
+      click_button "Add"
+    end
+
+    expect(page).to have_content "The related content you are adding already exists."
+  end
+
   scenario "related content can be scored positively" do
     related_content = create(:related_content, parent_relationable: relationable, child_relationable: related1, author: build(:user))