Get rid of n+1 queries in learning_resources API

[mbertrand]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/8259

Description (What does it do?)

• Prefetches learning resource children to avoid n+1 queries

How can this be tested?

Assuming you already have a bunch of programs and podcasts in your database:

• On the main branch, go to:
  • http://open.odl.local:8065/api/v1/learning_resources/?limit=100&resource_type=program&foo=bar1
  • http://open.odl.local:8065/api/v1/learning_resources/?limit=100&resource_type=podcast&foo=bar1

Check the logs, you'll likely see lots of n+1 queries

• Switch to this branch and go to:
  • http://open.odl.local:8065/api/v1/learning_resources/?limit=100&resource_type=program&foo=bar2
  • http://open.odl.local:8065/api/v1/learning_resources/?limit=100&resource_type=podcast&foo=bar12

You should no longer see any n+1 queries, just "Potential unnecessary eager load" which is okay. The children attribute of the results for each resource should be identical with the main branch results.

[abeglova]

👍