Fix error when editing an issuable with a task list

app/services/todo_service.rb

@@ -161,11 +161,16 @@ class TodoService
 
   def update_issuable(issuable, author)
     # Skip toggling a task list item in a description
-    return if issuable.tasks? && issuable.updated_tasks.any?
+    return if toggling_tasks?(issuable)
 
     create_mention_todos(issuable.project, issuable, author)
   end
 
+  def toggling_tasks?(issuable)
+    issuable.previous_changes.include?('description') &&
+      issuable.tasks? && issuable.updated_tasks.any?
+  end
+
   def handle_note(note, author)
     # Skip system notes, and notes on project snippet
     return if note.system? || note.for_snippet?

spec/services/todo_service_spec.rb

@@ -108,7 +108,8 @@ describe TodoService, services: true do
         should_not_create_todo(user: john_doe, target: confidential_issue, author: john_doe, action: Todo::MENTIONED)
       end
 
-      it 'does not create todo when when tasks are marked as completed' do
+      context 'issues with a task list' do
+        it 'does not create todo when tasks are marked as completed' do
           issue.update(description: "- [x] Task 1\n- [X] Task 2 #{mentions}")
 
           service.update_issue(issue, author)
@@ -120,6 +121,13 @@ describe TodoService, services: true do
           should_not_create_todo(user: member, target: issue, action: Todo::MENTIONED)
           should_not_create_todo(user: non_member, target: issue, action: Todo::MENTIONED)
         end
+
+        it 'does not raise an error when description not change' do
+          issue.update(title: 'Sample')
+
+          expect { service.update_issue(issue, author) }.not_to raise_error
+        end
+      end
     end
 
     describe '#close_issue' do
@@ -285,7 +293,8 @@ describe TodoService, services: true do
         expect { service.update_merge_request(mr_assigned, author) }.not_to change(member.todos, :count)
       end
 
-      it 'does not create todo when when tasks are marked as completed' do
+      context 'with a task list' do
+        it 'does not create todo when tasks are marked as completed' do
           mr_assigned.update(description: "- [x] Task 1\n- [X] Task 2 #{mentions}")
 
           service.update_merge_request(mr_assigned, author)
@@ -297,6 +306,13 @@ describe TodoService, services: true do
           should_not_create_todo(user: member, target: mr_assigned, action: Todo::MENTIONED)
           should_not_create_todo(user: non_member, target: mr_assigned, action: Todo::MENTIONED)
         end
+
+        it 'does not raise an error when description not change' do
+          mr_assigned.update(title: 'Sample')
+
+          expect { service.update_merge_request(mr_assigned, author) }.not_to raise_error
+        end
+      end
     end
 
     describe '#close_merge_request' do