Encountered a problem for grading Essay Questions.
To reproduce the problem:
0. Edit the essay question type so that the method is_usable_by_random returns true.
1. Add a random question to a quiz and select category X for the random question.
2. In category X only have Essay Question(s)
3. Take the quiz as a student.
4. Go to the Manual Grading Tab as a Teacher and you will find that there are no essay questions to grade.
This is because of how the code checks for essay questions in the quiz. Go to line 256 in grading.php to see how it does this.
The random question type causes a lot of problems for grading Essay Questions because if the random question is pulling from Essays and non-essays then not every student will answer the question.
So, the method for how the grading code looks for essay questions in a quiz and the interface for grading essay questions need to be altered. I'm not entirely sure how random questions work so I am stuck on this problem at the present time.
Another solution is to add ESSAY to the excludedtypes array in random question type.