Add create_from_super method and test

[bckohan]

This PR replaces #65

It needs some more work. I would like it to support casting across multiple levels of hierarchy.

The discussion in #65 and the details of this long standing Django ticket should be considered.

Fixes #296

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.75%. Comparing base (1d1d2d9) to head (6cd80a3).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
+ Coverage   76.44%   76.75%   +0.30%     
==========================================
  Files          22       22              
  Lines        1350     1368      +18     
  Branches      211      214       +3     
==========================================
+ Hits         1032     1050      +18     
  Misses        250      250              
  Partials       68       68              

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[Copilot]

Pull request overview

This PR adds a create_from_super method to the PolymorphicManager class that enables "recasting" an existing polymorphic model instance to a more specific subclass type. The method takes an instance of a parent class and creates a child class instance with the same database ID and inherited data, effectively converting the object down the inheritance hierarchy.

Key Changes:

• Adds create_from_super() method to PolymorphicManager for model recasting functionality
• Implements validation to ensure objects can only be cast to direct child classes
• Creates new test suite to verify recasting behavior across multiple inheritance levels

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 12 comments.

File	Description
src/polymorphic/managers.py	Adds create_from_super manager method with validation, database operations, and content type updates to enable polymorphic recasting
src/polymorphic/tests/test_recasting.py	Adds test suite covering successful recasting from Model2C→Model2D and error cases for non-adjacent inheritance levels

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[bckohan]

It might be best to solve this upstream in Django - a solution to this would also work for polymorphic models because they are just multi-table models with a fancy manager.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

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