enforce_atomic decorator for commit callbacks to workaround MySQL limitations

[Joshun]

Possible solution to an issue noticed here with the MySQL implementation of the Account and Leg triggers We were using transaction.on_commit to call stored procedures after the transaction had been committed, which successfully raise an error but do not probably rollback changes since the commit has already taken place.

Mostly an experiment, there are almost certainly reasons why this approach may be unsuitable.

Since MySQL doesn't have deferred triggers, the only way I can see of us enforcing these triggers is to call them from Python code.

As you can see some of the tests fail, this is because this approach currently only works if functions are decorated with @enforce_atomic

[codecov-commenter]

Codecov Report

Patch coverage: 89.47% and project coverage change: -0.05% :warning:

Comparison is base (322c264) 92.95% compared to head (213d807) 92.90%. Report is 5 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
- Coverage   92.95%   92.90%   -0.05%     
==========================================
  Files          59       59              
  Lines        3817     3848      +31     
  Branches      245      254       +9     
==========================================
+ Hits         3548     3575      +27     
- Misses        224      226       +2     
- Partials       45       47       +2     

Files Changed	Coverage Δ
hordak/models/core.py	96.88% <88.88%> (-1.35%)	:arrow_down:
hordak/tests/models/test_core.py	100.00% <100.00%> (ø)

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[autoantwort]

Maybe one could create a feature request at the django project for a pre_commit callback. Edit: Ok that would not help since there are no django transactions in auto commit mode.

[Joshun]

Maybe one could create a feature request at the django project for a pre_commit callback. Edit: Ok that would not help since there are no django transactions in auto commit mode.

I was thinking if the change was made in django-hordak, you could throw an exception when attempting to save a Leg object outside of an enforce_commit transaction (I could add a check to the overriden .save() method - preventing autocommit etc. and someone doing Leg.save() without doing a transaction. But I appreciate this might seem a bit too hacky?

[autoantwort]

And that does not cover all cases. You can change an object without calling save. For example by calling update

[adamcharnock]

FWIW: I'm not keen on adding complexity to the core Hordak codebase in order to support MySQL's limitations (which includes the current use of transaction.on_commit). My (perhaps hard-nosed) attitude is that if people deploying Hordak want advanced database features then I would prefer they work towards using a database that supports these features.

Adding complexity and workarounds to Hordak's core feels like pushing in the wrong direction to me.

I'm open to conversation on this, but I'm currently leaning towards Hordak having 'MySQL support with caveats'.