Proper handling of custom m2m relationships

[PiotrKurnik]

django-simple-history does not work with custom through tables for m2m relations.

Description

Code snippet below illustrates the issue. There are assumptions to the name of the fields in through tables for m2m tables.
For the default through tables it works, but for the custom ones it does not.

If you try to save object of PageArticle you will see:

django.core.exceptions.FieldError: Cannot resolve keyword 'pagearticle' into field. Choices are: article, article_id, id, tag, tag_id

from django.db import models
from simple_history.models import HistoricalRecords

class Tag(models.Model):
    name = models.CharField(max_length=100)

class ArticleTag(models.Model):
    article = models.ForeignKey("PageArticle", on_delete=models.CASCADE)
    tag = models.ForeignKey(Tag, on_delete=models.CASCADE)

class PageArticle(models.Model):
    title = models.CharField(max_length=100)
    article_tags = models.ManyToManyField(Tag, through=ArticleTag)

    history = HistoricalRecords(m2m_fields=[
        article_tags,
    ])

Related Issue

#1079

Motivation and Context

We need to use model introspection to find the correct field name and not to relate on model name.

How Has This Been Tested?

Unit test has been added.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #1093 (eb732e1) into master (309609e) will decrease coverage by 0.15%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1093      +/-   ##
==========================================
- Coverage   97.23%   97.08%   -0.16%     
==========================================
  Files          23       23              
  Lines        1231     1236       +5     
  Branches      199      202       +3     
==========================================
+ Hits         1197     1200       +3     
  Misses         16       16              
- Partials       18       20       +2     

Impacted Files	Coverage Δ
simple_history/models.py	96.34% <66.66%> (-0.38%)	⬇️

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[legau]

add the case where m2m has through_fields

https://github.com/django/django/blob/main/django/db/models/fields/reverse_related.py#L383

[lazarust]

@PiotrKurnik I saw you tagged #1079 on this issue but this PR but it doesn't seem to allow adding custom table names for each of the m2m fields. Am I missing how to set the custom table names?

[ddabble]

Looks like #1218 fixes this issue - at least the tests added in this PR and the example provided in #1094 now work without any errors, and they stop working when reverting the changes made in models.py - so I'll close this.

Please reopen if it turns out that the issue was not resolved 🙂