Fix: DjangoOptimizerExtension corrupts nested objects' fields' prefetch objects

[aprams]

Description

This PR aims to resolve issue #379 .

I included a reproducible test and a suggested fix, where the Prefetch object gets deepcopied to avoid the side effects from add_prefix.

Considerations:

• There were multiple ways to do this, including copying the OptimizerStore at a higher level in the execution stack, but this should be the least invasive one
• I used deepcopy now as Prefetch didn't offer an easier way of copying the object that I know of, I'm happy about suggestions on improving this

I'm also very open for improving the test case included. If you know a better way on how to do it without the custom type setup, please let me know.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

• DjangoOptimizer prefetch_related on nested objects modifies prefetch #379

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• 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).

I love the work you do here, thanks a lot for the really awesome work! ❤️

[bellini666]

Thanks for this! :)

The -1 is due to the copy vs deepcopy comment I've made. Solving it and we should be good to merge

[bellini666]

Hrm, the only solution I can think right now would be to create a Prefetch object by hand, but then we would need to update it in case it gets changed in django.

The only question here would be: Any reason to use deepcopy instead of copy? Unless I'm missing something, a copy should be enough to fix the issue here

[aprams]

Yeah, I tried instantiating directly, creating the Prefetch object by hand and it's also not super trivial with the internal levels and protected variables. Kind of a messy hard-to-copy class.

I don't really have a good reason for deepcopy, fair point, I changed it to copy.copy and the test still works :)

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (a17b51b) 87.98% compared to head (a6695b5) 87.99%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #380   +/-   ##
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  Coverage   87.98%   87.99%           
=======================================
  Files          33       33           
  Lines        2971     2973    +2     
=======================================
+ Hits         2614     2616    +2     
  Misses        357      357           

Files	Coverage Δ
strawberry_django/optimizer.py	89.13% <100.00%> (+0.06%)	⬆️

... and 2 files with indirect coverage changes

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