Fix crash with read-only polymorphic sub-serializer.

[ngnpope]

If all fields are read-only when using split requests the component is pruned from the registry and if no longer available. If that is the case, we should just skip that component.

[codecov]

Codecov Report

Merging #542 (579fbf8) into master (d0dfdb4) will decrease coverage by 0.02%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
- Coverage   98.71%   98.69%   -0.03%     
==========================================
  Files          57       57              
  Lines        6302     6343      +41     
==========================================
+ Hits         6221     6260      +39     
- Misses         81       83       +2     

Impacted Files	Coverage Δ
tests/contrib/test_rest_polymorphic.py	98.64% <95.45%> (-1.36%)	⬇️
drf_spectacular/contrib/rest_polymorphic.py	94.73% <100.00%> (+0.61%)	⬆️
drf_spectacular/openapi.py	97.07% <0.00%> (-0.13%)	⬇️
tests/test_regressions.py	100.00% <0.00%> (ø)
drf_spectacular/plumbing.py	97.60% <0.00%> (+0.01%)	⬆️

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[ngnpope]

Thanks for the tidy up! Glad I was able to track this one down though 🙂

[tfranzel]

nice find @ngnpope!

I also added a fix for PolymorphicProxySerializer, which had the same issue. Condensed down the test to that specific aspect only, as I had to do it again for the other case.

[tfranzel]

i see that you pretty much use all the features in spectacular. glad to see that! i'm quite amazed at how seasoned code that is likely used by a bunch of people still can contain simple bugs.