feat: filter submissions by payload content

[quesebifurcan]

Add a dynamic filter to SubmissionViewSet and EntityViewSet.

Any query parameter prefixed by "payload__" will filter submissions
based on the contents of e.g. Submission.payload.

Example:
The URL "/submissions?payload__a__b__c=1" will yield
the queryset

models.Submission.objects.filter(payload__a__b__c=1)

and return a list of submissions with a JSON payload like

'{"a": {"b": {"c": 1}}}'

Note that it is possible to compare not just strings, but
numbers, lists and objects as well.

[obdulia-losantos]

This line will remove previous filters executed by the SubmissionFilter class like the project filter 😱
The correct line is queryset = self.queryset 😛

[quesebifurcan]

Oh thanks, I thought this ran before the other filters. I'll take a look.

[obdulia-losantos]

Maybe you are right, but imagine that the default queryset is something like this:

    queryset = Project.objects \
                      .prefetch_related('xforms', 'xforms__media_files') \
                      .order_by('name')

or

    queryset = models.Project \
                     .objects \
                     .values('id', 'name', 'created') \
                     .annotate(
                         first_submission=Min('submissions__created'),
                         last_submission=Max('submissions__created'),
                         submissions_count=Count('submissions__id', distinct=True),
                         entities_count=Count('submissions__entities__id', distinct=True),
                     )

🤷‍♀️

[quesebifurcan]

See docs here: https://www.django-rest-framework.org/api-guide/filtering/#overriding-the-initial-queryset

[quesebifurcan]

Yes, agreed it's much clearer to use self.queryset. Fixing.