Fix infinite recursion bug when using only()

[bckohan]

Closes #334

[Copilot]

Pull request overview

This PR fixes an infinite recursion bug (issue #334) that occurred when using refresh_from_db() with limited fields on polymorphic models. The bug was caused by the only() method not including polymorphic_ctype_id in the field list, which prevented the polymorphic machinery from determining the correct model type.

Key changes:

• Modified PolymorphicQuerySet.only() to use a set for deduplication and always include polymorphic_ctype_id
• Added comprehensive test case reproducing the bug scenario
• Added codecov configuration for better coverage reporting

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/polymorphic/query.py	Fixed only() method to use a set for field deduplication and ensure polymorphic_ctype_id is always included
src/polymorphic/tests/test_orm.py	Added test case test_infinite_recursion_with_only that reproduces the bug scenario with refresh_from_db()
src/polymorphic/tests/models.py	Added RecursionBug model with __init__ method that triggers the bug when fields are partially loaded
src/polymorphic/tests/migrations/0001_initial.py	Regenerated migration to include the new RecursionBug model and reordered dependencies
codecov.yml	Added codecov configuration to fix source path mapping in coverage reports

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.77%. Comparing base (253abe7) to head (a3c93f5).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #739      +/-   ##
==========================================
+ Coverage   76.75%   76.77%   +0.01%     
==========================================
  Files          22       22              
  Lines        1368     1369       +1     
  Branches      214      214              
==========================================
+ Hits         1050     1051       +1     
  Misses        250      250              
  Partials       68       68              

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