Deleting a project with active annotators leaves those annotators ineligible to join another project

[ianroberts]

Describe the bug

If an "in-progress" annotation project is deleted, any active annotator users who are assigned to that project are left permanently ineligible to be assigned to any other projects.

To Reproduce

Steps to reproduce the behavior:

1. Create a project
2. Assign an annotator to that project
3. Delete the project
4. Attempt to assign the same annotator to a different project - they are not listed in the eligible annotators list

Expected behavior

Deleting a project should free up the annotators who were assigned to that project to be able to annotate other projects.

Additional context

The root cause of this problem appears to be that the foreign key constraints on AnnotatorProject are set to simply set the project field to NULL when the referenced Project is deleted, rather than deleting the whole AnnotatorProject assignment:

gate-teamware/backend/models.py

Lines 841 to 842 in 8b54bc1

	annotator = models.ForeignKey(get_user_model(), on_delete=models.SET_NULL, null=True, blank=True)
	project = models.ForeignKey(Project, on_delete=models.SET_NULL, null=True, blank=True)

This means that the user is not assigned to a project, but they are still considered as "active" in rpc.get_possible_annotators:

gate-teamware/backend/rpc.py

Lines 715 to 719 in 8b54bc1

	# get a list of IDs of annotators that is currently active in any project
	active_annotators = User.objects.filter(annotatorproject__status=AnnotatorProject.ACTIVE).values_list('id', flat=True)
	project_annotators = project.annotators.all().values_list('id', flat=True)
	# Do an exclude filter to remove annotator with the those ids
	valid_annotators = User.objects.filter(is_deleted=False).exclude(id__in=active_annotators).exclude(id__in=project_annotators)

and thus not available to be assigned to any other project. When this happened to @iabufarha in production I had to delete the offending row from the backend_annotatorproject table directly in the database to unblock them.

Possible fixes

1. use on_delete=models.CASCADE so Teamware deletes all AnnotatorProject assignments for this project when the project is deleted, and/or
2. perform checks in rpc.delete_project and disallow deletion of a project that has active annotators. The manager would need to mark all annotators as "complete" or "rejected" before being allowed to delete the project.

[ianroberts]

The same issue manifests itself the other way around if a user deletes their account while they are assigned to a project:

gate-teamware/backend/models.py

Lines 841 to 842 in 5e7b5c4

	annotator = models.ForeignKey(get_user_model(), on_delete=models.SET_NULL, null=True, blank=True)
	project = models.ForeignKey(Project, on_delete=models.SET_NULL, null=True, blank=True)

In this case we end up with a row in the AnnotatorProject join table where the annotator_id is NULL, which breaks the project management view since the rpc call getting the project annotators assumes that ap.annotator is not None:

gate-teamware/backend/rpc.py

Lines 728 to 733 in 5e7b5c4

	for ap in project_annotators:
	output.append({
	**serializer.serialize(ap.annotator, {"id", "username", "email"}),
	**serializer.serialize(ap, exclude_fields={"annotator", "project"}),
	**ap.get_stats()
	})

We can't stop users from deleting their accounts, so this one probably does need to be an on_delete=CASCADE.