I have a model that represents interdependent tasks. A task cannot be
started until all of its prerequisite tasks have been completed. I
want to find all the tasks which are ready to start, i.e. all their
prerequisite tasks (if any) have been completed. This is the relevant
part of the model:
class Task(models.model):
TASK_STATUS_CHOICES = (
(u"P", u'Pending'),
(u"A", u'Assigned'),
(u"C", u'Complete'),
(u"F", u'Failed')
)
status = models.CharField(max_length=1,
choices=TASK_STATUS_CHOICES)
prerequisites = models.ManyToManyField('self', symmetrical=False,
related_name="dependants")
I want to write a query that will return the set of all tasks for
which *all* prerequisite tasks are complete. So far I can only figure
out how to get the set of all tasks for which *any* prerequisite task
is complete:
Task.objects.filter(prerequisites__status=u"C")
How do I find the set of tasks for which all prerequisite tasks are
complete? Do I need to resort to writing SQL? Is there some way to
apply a filter before an annotation? For example, here I can annotate
each task with its number of prerequisites:
Task.objects.annotate(prereq_count=Count('prerequisites'))
Is there a way to make this aggregation count only the prerequisite
tasks that are not complete, i.e. to annotate each task with a count
of the number of prerequisite tasks it has for which status is not
equal to "C"?
(Note: I have also asked this question on StackOverflow. Here is the
link:
http://stackoverflow.com/questions/1516795/in-django-how-do-i-filter-based-on-all-entities-in-a-manytomany-relation-instead
)
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