Add AUDITLOG_EXCLUDE_REVERSE_RELATIONS to avoid logging reverse relation fields

[peymanahmadifar]

Fixes issue #551 — Reverse relations are being logged.

Summary:
When computing model diffs we used Model._meta.get_fields() which includes
auto-created reverse relation fields. Those fields can trigger extra DB queries
(if not prefetched) and mutate instance._state.fields_cache as a side-effect.

This PR:

• Adds a small opt-in setting AUDITLOG_EXCLUDE_REVERSE_RELATIONS (default False).
• When enabled, reverse relation fields (auto_created & not concrete) are filtered out
  from the fields used to calculate diffs.

Notes:

• Behaviour is opt-in to remain backward compatible.
• I added a minimal documentation entry under configuration (docs).
• I also added a (minimal) test verifying that reverse relations are excluded when the setting is enabled.

Please let me know if you'd prefer to make this opt-out instead of opt-in,
or if you'd rather that the default behavior be changed. Happy to update the PR.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.58%. Comparing base (85056b7) to head (952924f).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
auditlog/diff.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
- Coverage   95.81%   95.58%   -0.24%     
==========================================
  Files          34       34              
  Lines        1195     1200       +5     
==========================================
+ Hits         1145     1147       +2     
- Misses         50       53       +3     

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[hramezani]

Thanks @peymanahmadifar for the PR.

Please add tests and a changelog entry.

@2ykwang Please review the PR

[2ykwang]

thanks for the patch! @peymanahmadifar
could you add test cases for this change?

[2ykwang]

Suggested change

	.. versionadded:: 3.2.2
	.. versionadded:: 3.4.0

[2ykwang]

I think the reverse-relation filtering belongs in track_field rather than model_instance_diff
That keeps the “what to track” policy in one place

def track_field(field):
    """
    Returns whether the given field should be tracked by Auditlog.

    Untracked fields are many-to-many relations and relations to the Auditlog LogEntry model.

    :param field: The field to check.
    :type field: Field
    :return: Whether the given field should be tracked.
    :rtype: bool
    """