fix(optimizer): handle existing select_related in querysets

[taobojlen]

Description

The strawberry-django optimizer can automatically add select_related and only clauses to a queryset. However, it currently fails if you enable the only optimization while using querysets that already have select_relateds on them.

We use strawberry at work, and have a model like:

class MyManager(models.Manager):
  def get_queryset(self):
    return super().get_queryset().select_related("foo")

class MyModel(models.Model):
  objects = MyManager()
  foo = models.ForeignKey(Foo, on_delete=models.CASCADE)

This causes issues with the optimizer if you try to execute a graphQL query that doesn't include foo: you get errors like Field cannot be both deferred and traversed using select_related at the same time. We've actually been running without the only optimization for a long time for this reason.

This PR adds some code to the optimizer that inspects the queryset for any existing select_relateds, and includes them in the .only() call.

Types of Changes

• [ ] Core
• [x] Bugfix
• [ ] New feature
• [ ] Enhancement/optimization
• [ ] Documentation

Issues Fixed or Closed by This PR

N/A, there's no issue for this, though it related to this old message in the Strawberry Discord.

Checklist

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [ ] I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[bellini666]

Note: this is a draft PR! I am testing this out on our code, but I figured I'd open this draft so you're aware :)

Let me know once it is ready to be reviewed :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.63%. Comparing base (ff50d0a) to head (973c687).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #515 +/- ## ========================================== + Coverage 88.59% 88.63% +0.03% ========================================== Files 40 40 Lines 3402 3413 +11 ========================================== + Hits 3014 3025 +11 Misses 388 388 ```

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[taobojlen]

apologies for the delay here @bellini666 ! this is now ready for review :)