fix: Calculate monthly repayment precisely to the fraction of a currency unit, turning rounding up into an optional feature.

[bosue]

Before:

Now also possible:

And this is how it works, UI-wise:

RPReplay_Final1695066171.mov

Fixes #31. Includes some minor UI improvements and some minor refactoring of related code.

Full Loan Repayment Schedules for comparison:
Before.pdf
With PR (rounded up).pdf
With PR (to the cent).pdf

[bosue]

A few failing tests, no big deal, but nice we have coverage!
Will set up a local test environment, fix the bug, amend and expand the tests and come back. ;)

[bosue]

The business logic related test fails need to be tackled in the end, that’s fine, these basically is the feature rather than a bug, though of course both need to be tested to work right:

  File "/home/runner/frappe-bench/apps/lending/lending/loan_management/doctype/loan/test_loan.py", line 130, in test_loan
    self.assertEqual(loan_repayment_schedule.monthly_repayment_amount, 15052)
AssertionError: 15051.72264885 != 15052

But these other update_after_submit related fails are almost driving me crazy:

  File "/home/runner/frappe-bench/apps/lending/lending/loan_management/doctype/loan/test_loan.py", line 415, in test_security_shortfall
    loan_application = create_loan_application(
  File "/home/runner/frappe-bench/apps/lending/lending/loan_management/doctype/loan/test_loan.py", line 1237, in create_loan_application
    loan_application.save()
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 331, in save
    return self._save(*args, **kwargs)
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 372, in _save
    self.validate_update_after_submit()
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/document.py", line 859, in validate_update_after_submit
    self._validate_update_after_submit()
  File "/home/runner/frappe-bench/apps/frappe/frappe/model/base_document.py", line 1016, in _validate_update_after_submit
    frappe.throw(
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 570, in throw
    msgprint(
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 542, in msgprint
    _raise_exception()
  File "/home/runner/frappe-bench/apps/frappe/frappe/__init__.py", line 491, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.UpdateAfterSubmitError:  Not allowed to change <strong>Monthly Repayment Amount</strong> after submission from <strong>89552.027429802</strong> to <strong>89552.02742980218</strong>

We shouldn‘t be changing anything on update after submit. I tried almost everything without any success. But specifically for updates after submit, in get_repayment_details() I can‘t even evaluate self.docstatus nor can I throw a frappe.msgprint() for debugging.
When skipping the method by an early unconditional return, the problem is gone. So even on upon update after submit, get_repayment_details() is getting called and does stuff.
But how can I conditionally skip get_repayment_details() only in the case we‘re updating a submitted doc, if for some reason or other I can‘t reliably evaluate self.docstatus?

This stubborn problem now cost me more time and nerves than all of the actual feature. I would be really grateful for some advise how to tackle this, @deepeshgarg007 or @anandbaburajan or someone else who happened to hit a similar problem.

Thank you so much! 🙏