Fix ZeroDivisionError in unit conversion causing recipe retrieval failure

[freakadings]

Tandoor Version

2.2.6

Setup

Docker / Docker-Compose

Reverse Proxy

Nginx Proxy Manager (NPM)

Other

psql16

Bug description

When clicking on certain recipes, the API returned an internal server error due to a division by zero in the unit conversion helper.

I traced the culprit down bychecking the "tandoor"-database via:

SELECT * FROM public.cookbook_unitconversion
WHERE converted_amount = 0 OR converted_amount IS NULL;

identified where "converted_amount" was "0"

entered the id at the end of the unit-convertion url:
http://tandoor.local/edit/UnitConversion/170
and entered a value above zero via the webui of tandoor.

Which fixed the error and i could browse through tandoor as usual.

I checked if I could enter it again and since i could, it seems there's no safeguard against converted_amount being zero (or null).

It's strange because the conversion was nothing i added recently. Probably the error appeared after an update?

Happy to provide more logs or database info as needed.

Relevant logs

MainThread ERROR 2025-09-25 21:39:43,450 django.request Internal Server Error: /api/recipe/57/
Traceback (most recent call last):
  File "/opt/recipes/venv/lib/python3.13/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
  File "/opt/recipes/venv/lib/python3.13/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/opt/recipes/venv/lib/python3.13/site-packages/django/views/decorators/csrf.py", line 65, in _view_wrapper
    return view_func(request, *args, **kwargs)
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/viewsets.py", line 125, in view
    return self.dispatch(request, *args, **kwargs)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/views.py", line 515, in dispatch
    response = self.handle_exception(exc)
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/views.py", line 475, in handle_exception
    self.raise_uncaught_exception(exc)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/views.py", line 486, in raise_uncaught_exception
    raise exc
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/views.py", line 512, in dispatch
    response = handler(request, *args, **kwargs)
  File "/opt/recipes/venv/lib/python3.13/site-packages/drf_spectacular/drainage.py", line 207, in wrapped_method
    return method(self, request, *args, **kwargs)
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/mixins.py", line 56, in retrieve
    return Response(serializer.data)
                    ^^^^^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 573, in data
    ret = super().data
          ^^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 251, in data
    self._data = self.to_representation(self.instance)
                 ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 540, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
                            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 716, in to_representation
    self.child.to_representation(item) for item in iterable
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 540, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
                            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 716, in to_representation
    self.child.to_representation(item) for item in iterable
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/serializers.py", line 540, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
                            ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/opt/recipes/venv/lib/python3.13/site-packages/rest_framework/fields.py", line 1870, in to_representation
    return method(value)
  File "/opt/recipes/cookbook/serializer.py", line 976, in get_conversions
    for c in uch.get_conversions(obj):
             ~~~~~~~~~~~~~~~~~~~^^^^^
  File "/opt/recipes/cookbook/helper/unit_conversion_helper.py", line 120, in get_conversions
    r = self._uc_convert(c, ingredient.amount, ingredient.unit, ingredient.food)
  File "/opt/recipes/cookbook/helper/unit_conversion_helper.py", line 142, in _uc_convert
    return Ingredient(amount=amount * (uc.base_amount / uc.converted_amount), unit=uc.base_unit, food=food, space=self.space)
                                       ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
decimal.DivisionByZero: [<class 'decimal.DivisionByZero'>]

[vabene1111]

thanks, I changed a few things about the conversion helper and likely did not test with a 0 value. I will implement to fixes

• do not allow 0 values in the frontend
• fix it in the helper to not cause errors
• add a test case

thanks for the well researched report