Teacher cannot rate database activities

Bug report after QA testing for MDLQA-1840. Teacher is only able to rate the first entry in the database activity. All subsequent entries produce the error "Rating of items not allowed!"

Steps to reproduce:

1. Login as a teacher, update the database activity and select 'Count of ratings' as ratings aggregate type and 10 as the scale.
2. Rate each of the database activity entries.

Note that you can probably rate the first entry, but subsequent entries produce an error. It should also be noted that for the first entry, which was rated successfully, the grade did show up in the grade book for that student.