[issue-983] querying history, convert to instance, chase foreign key on history_date

[jeking3]

Description

This fixes an issue whereby:

1. User queries history directly.
2. User converts Historical<> to instance with instance property.
3. Original query (from 3.1) was not with history.as_of so it did not have _as_of property
4. instance failed

The fix:

When _history._as_of is not present use the history.history_date field instead
when chasing foreign keys.

This closes #983

In the end, if you did not query history with as_of, and you end up with a historical instance, if you call .instance on it, the HistoricalForeignKey chasers will use a fallback of the history_date of the record that made the instance when chasing relations! This was an easy fix and an easy win, and makes django-simple-history more powerful.

[codecov]

Codecov Report

Merging #984 (c806622) into master (720fd8a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #984   +/-   ##
=======================================
  Coverage   97.64%   97.64%           
=======================================
  Files          23       23           
  Lines        1147     1147           
  Branches      222      222           
=======================================
  Hits         1120     1120           
  Misses         12       12           
  Partials       15       15           

Impacted Files	Coverage Δ
simple_history/models.py	97.77% <100.00%> (ø)

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[tim-schilling]

This makes sense to me. Good catch!

[LuxsSoft]

This seems not fully resolved.

I had the mentioned error before version 3.2.0, now i can get a record with something like historicalOrgInstance.history.latest().instance and also historicalOrgInstance.history.latest().prev_record.instance. However getting the instance that way always returns an empty set of historical foreign key models.

It only works by getting the instance through an as_of() query, then the reverse relationships models show up through the related name eg.
historicalOrgInstance.historical_participants.all()

[LuxsSoft]

More specifically here is what i am trying to do

t1: org with participant 1

org = HistoricalOrg(name="original")
part1 = HistoricalParticipant.objects.create(name="p1", org=org)
before_mod = timezone.now()

t2: org with participant 2

org.name="modified"
org.save()
org.participants.all().delete()
part2 = HistoricalParticipant.objects.create(name="p2", org=org)

now what works as expected:

org.as_of(before_mod).participants.all() # returns part1
org.as_of(before_mod).name # returns "original"
org.as_of(timezone.now().participants.all() # returns part2
org.as_of(timezone.now()).name # returns "modified"
org.history.most_recent().participants.all() # returns part2
org.history.most_recent() # returns "modified"

what is weird when i want to work with the helpers:

org.history.first().instance.name # returns "modified" as expected
org.history.first().instance.participants.all() returns part1 BUT part2 is expected
org.history.first().prev_record.instance.name # returns "original" as expected
org.history.first().prev_record.instance.participants.all() # returns empty HistoricalQuerySet [] BUT part1 is expected

[tim-schilling]

@LuxsSoft it would be best to open up a new issue and write a draft PR with the tests that show the problem.

[LuxsSoft]

@LuxsSoft it would be best to open up a new issue and write a draft PR with the tests that show the problem.

ok i tried to make the case