Performance: LDPViewSet can prefetch the appropriate fields

In my performance testing on 30/10/2020, I found that by using Django's prefetch_related utility during the LDPViewSet.get_queryset with the right fields, I was able to reduce the serialization time on /users/ of 9,000 resources from avg. 22s to avg. 14.5s, and reduce the number of database hits from around 9,000 to < 50 (figure 8)

The issue then is to select the appropriate fields automatically (as doing it manually reintroduces an old issue of tight-coupling between DjangoLDP packages)

The optimum fields for /users/ was:

('projects', 'projects__user', 'projects__project', 'circles', 'jobOffers', 'inbox', 'groups', 'skills', 'communities', 'communities__user', 'communities__community', 'chatProfile', 'profile', 'account')

Which can be broken up into the sub-groups

Foreign keys: a priori we can fetch all of these without any significant overhead

('chatProfile', 'profile', 'account')

Through-model foreign keys: likewise these can be fetch-all, and automatically deduced from the through model, to a certain depth

('projects__user', 'projects__project', 'communities__user', 'communities__community')

Selected nested-fields: this is the tricky part. We're after all the nested fields that will be serialized

Implementation

LDPViewSet should generate (via a new method) automatically the prefetch fields described above

• working with cls.depth
• (unit) testing