+ def test_display_name_user_id_new
+ existing_user = create(:user)
+ user = build(:user)
+
+ user.display_name = "user_#{existing_user.id}"
+ assert_not user.valid?, "user_<id> name is valid for existing user id when it shouldn't be"
+
+ user.display_name = "user_#{existing_user.id + 1}"
+ assert_not user.valid?, "user_<id> name is valid for new user id when it shouldn't be"
+ end
+
+ def test_display_name_user_id_rename
+ existing_user = create(:user)
+ user = create(:user)
+
+ user.display_name = "user_#{existing_user.id}"
+ assert_not user.valid?, "user_<id> name is valid for existing user id when it shouldn't be"
+
+ user.display_name = "user_#{user.id}"
+ assert_predicate user, :valid?, "user_<id> name is invalid for own id, when it should be"
+ end
+