Document post_save signal behavior during loaddata (#502)

[JohananOppongAmoateng]

Users reported that when using loaddata to load fixtures for polymorphic models, post_save signal handlers receive instances with:

Mismatched pk and id fields
Missing parent class attributes (empty values)
This only occurs during loaddata, not during normal instance creation.

Root Cause
This is expected Django behavior for multi-table inheritance during deserialization, not a bug in django-polymorphic. Closes #502

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.40%. Comparing base (0d30c86) to head (fa458d2).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #792      +/-   ##
==========================================
- Coverage   83.44%   83.40%   -0.04%     
==========================================
  Files          26       27       +1     
  Lines        1649     1621      -28     
  Branches      264      255       -9     
==========================================
- Hits         1376     1352      -24     
+ Misses        207      206       -1     
+ Partials       66       63       -3     

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[bckohan]

This is looking good thanks @JohananOppongAmoateng

I assume this is discussed in the django documentation? Can we add a link to it using intersphinx references? There's a shortcut for pulling down the intersphinx objects using just:

just fetch-refs django

[JohananOppongAmoateng]

@bckohan added the refs

[bckohan]

On further consideration this is just normal Django behavior and I don't think it's our responsibility to document. The issue was converted to Q&A and answered.