New TodoService#todo_exists? method

app/services/slash_commands/interpret_service.rb

@@ -140,7 +140,7 @@ module SlashCommands
     condition do
       noteable.persisted? &&
       current_user &&
-      !TodosFinder.new(current_user).execute.exists?(target: noteable)
+      !TodoService.new.todo_exist?(noteable, current_user)
     end
     command :todo do
       @updates[:todo_event] = 'add'
@@ -149,7 +149,7 @@ module SlashCommands
     desc 'Mark todo as done'
     condition do
       current_user &&
-      TodosFinder.new(current_user).execute.exists?(target: noteable)
+      TodoService.new.todo_exist?(noteable, current_user)
     end
     command :done do
       @updates[:todo_event] = 'done'

app/services/todo_service.rb

@@ -154,6 +154,10 @@ class TodoService
     create_todos(current_user, attributes)
   end
 
+  def todo_exist?(issuable, current_user)
+    TodosFinder.new(current_user).execute.exists?(target: issuable)
+  end
+
   private
 
   def create_todos(users, attributes)

spec/lib/gitlab/slash_commands/dsl_spec.rb

@@ -51,7 +51,7 @@ describe Gitlab::SlashCommands::Dsl do
         { name: :one_arg, aliases: [:once, :first], description: '', params: ['The first argument'], noop: false, cond_block: nil },
         { name: :two_args, aliases: [], description: '', params: ['The first argument', 'The second argument'], noop: false, cond_block: nil },
         { name: :cc, aliases: [], description: '', params: [], noop: true, cond_block: nil },
-        { name: :wildcard, aliases: [], description: '', params: [], noop: false, cond_block: nil}
+        { name: :wildcard, aliases: [], description: '', params: [], noop: false, cond_block: nil }
       ]
     end
 

spec/services/todo_service_spec.rb

@@ -290,6 +290,18 @@ describe TodoService, services: true do
         should_create_todo(user: author, target: unassigned_issue, action: Todo::MARKED)
       end
     end
+
+    describe '#todo_exists?' do
+      it 'returns false when no todo exist for the given issuable' do
+        expect(service.todo_exist?(unassigned_issue, author)).to be_falsy
+      end
+
+      it 'returns true when a todo exist for the given issuable' do
+        service.mark_todo(unassigned_issue, author)
+
+        expect(service.todo_exist?(unassigned_issue, author)).to be_truthy
+      end
+    end
   end
 
   describe 'Merge Requests' do