Fetching attendances for large sections is slow

[smartspot2]

This should be investigated further, but when fetching attendances for large sections (for reference, I have a section of ~40 people), the request takes ~2 seconds to load. A similar load time can be observed when fetching the student roster list.

The same can be reproduced locally with a section of 30 people with all attendance objects populated from 3 months, where sections occur 2 times per week. The request time is fast without any attendance objects populated though.

This is likely due to a very inefficient query/filter in the viewset, but should be investigated further for the root cause and possible solutions.

[joshualiao]

The path for fetching attendances is inefficient.

The pseudocode is:

1. Filter requesting user's banned sections.
2. Get all sections, minus the banned sections, filtered on if the section matches the user's mentor, student, or coordinator status.
3. From this list of sections, find the one the request desires.
4. Serialize the SectionOccurrences attached to the section, and each sectionOccurrence is just the serialization of set of Attendances attached to each section occurrence
   -> Each attendance serialization pulls sectionOccurrence and student info, and return attendance object.

The easiest optimization should just be to pull the specified section ID first?