TICKET-574: Implement assignment caching

[Naragod]

Proposed Changes

Multiple individuals have reported slow MarkUs load times when viewing assignments, as TAs. Our investigation has found that we are running into the common N + 1 query problem. When loading the information displayed on the status column of the assignment index page, we are performing 2 calculations per assignment, each doing a join on the membership table. There are currently more than 300K rows on the membership table. This means that every assignment scans through 300K membership records.

If there are 10 assignments assigned to a particular TA, this means that there will be approximately 20 queries each scanning ~300K membership rows (we lack indexes as well) -> ~6 million row scans per page load.

To resolve this issue, we can preload all of the TA statistics and then return this information as a hash, where each key is associated to an assignment id and each value, to the status of that assignment, num_marked or num_assigned. This means we are doing a single expensive query and then presenting this data almost immediately when subsequently asked for it.

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[coveralls]

Pull Request Test Coverage Report for Build 21258432992

Details

• 69 of 69 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 91.654%

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Totals
Change from base Build 21224303427:	0.01%
Covered Lines:	44364
Relevant Lines:	47596

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💛 - Coveralls