[#4396] Objects API prefill plugin

[vaszig]

Closes #4396 partially

Changes

• Added necessary model field prefill_options to the FormVariable model (for storing all the needed configuration) and updated constraints according to new field
• Moved public prefill functions to prefill.service
• Added prefill functionality for ObjectsAPI

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • Checked copying a form
  • Checked import/export of a form
  • Config checks in the configuration overview admin page
  • Problem detection in the admin email digest is handled

• Release management

  • I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • Commit messages refer to the relevant Github issue
  • Commit messages explain the "why" of change, not the how

[codecov]

Codecov Report

Attention: Patch coverage is 92.41071% with 17 lines in your changes missing coverage. Please review.

Project coverage is 96.54%. Comparing base (751991d) to head (2890501).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...c/openforms/forms/api/serializers/form_variable.py	55.55%	7 Missing and 1 partial ⚠️
src/openforms/prefill/service.py	87.93%	3 Missing and 4 partials ⚠️
src/openforms/prefill/sources/user_defined.py	92.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4620      +/-   ##
==========================================
- Coverage   96.56%   96.54%   -0.03%     
==========================================
  Files         746      750       +4     
  Lines       25210    25340     +130     
  Branches     3322     3341      +19     
==========================================
+ Hits        24345    24465     +120     
- Misses        602      611       +9     
- Partials      263      264       +1     

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[sergei-maertens]

isn't this broken now because of the local import?

[vaszig]

I don't see a problem here in the tests and if I remove this the tests are failing.

[sergei-maertens]

I don't understand this one -

• ~USER_DEFINED implies COMPONENT (since there are only two options)
• ~EMPTY_PREFILL_PLUGIN implies that a plugin is selected.

this is the behaviour of the master branch - and here the check constraint says that then also the attribute must be set: ~EMPTY_PREFILL_ATTRIBUTE, but the code here allows/requires an empty attribute 🤔

shouldn't this combination be disallowed entirely? You then end up with the following options:

• no prefill configured at all (no plugin selected -> no attribute or options may be set)
• "previous" prefill configuration (plugin selected, attribute selected -> prefill options must be empty), can be used for both components and user defined vars
• "new" prefill configuration (plugin selected, options set -> attribute may not be set, source must be user defined)

I think this union member is not needed then?

[vaszig]

This will remain (if I am not mistaken) because of the comment below
#4620 (comment)

[sergei-maertens]

Unresolving this as a reminder so that we can discuss this at the end of the day to make sure I understand everything correctly!