django integer model field with float form input

Question

I am working with a django model which stores currency values as integers. i.e. GBP22.35 gets stored as 2235.

When that model gets rendered as a form, I need to be able to associate a widget with that integer field so can be edited as though it were a float value (i.e. to two decimal places - 22.35) and is validated as such. form.save() then needs to save the native integer value to the db.

I've tried creating a custom FormField/Widget pair which involves dividing by 100 in the widgets render and multiplying back up in the fields to_python method, but it all goes awry if there is an error on the form. The widget keeps re-dividing the value.

I realise this could be avoided by using float/decimal model fields but that is not an option in this case.

Is this something people have done before? Any pointers? Thanks.

Why not use FloatField ?? Ok, I see you don't have that option. — Raydel Miranda
– Raydel Miranda, Commented Mar 11, 2014 at 13:11
@RaydelMiranda Storing currency data in a FloatField is never an acceptable option. — lanzz
– lanzz, Commented Mar 11, 2014 at 13:19

Jarad · Accepted Answer · 2022-06-17 16:33:00Z

You could implement a subclass of IntegerField that handles that conversion behind the scenes:

import decimal
from django.db import models
from django.core import exceptions
from django.utils.translation import ugettext_lazy as _

class CentsField(models.IntegerField):
    empty_strings_allowed = False
    default_error_messages = {
        'invalid': _("'%(value)s' value must be a decimal number."),
    }
    description = _("Fixed-point number")

    __metaclass__ = models.SubfieldBase

    def to_python(self, value):
        if value is None or isinstance(value, decimal.Decimal):
            return value
        try:
            if isinstance(value, int):
                return decimal.Decimal(value) / 100
            else:
                return decimal.Decimal(value)
        except decimal.InvalidOperation:
            raise exceptions.ValidationError(
                self.error_messages['invalid'],
                code='invalid',
                params={'value': value},
            )

    def get_prep_value(self, value):
        return int(value * 100)

Should behave as DecimalField in your Django code, but as IntegerField in your database.

Update: simpler implementation derived from IntegerField instead of DecimalField; added validation as implemented in DecimalField.to_python

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