Improve error message when trying to insert a value that overflows DecimalField

[Floris den Hengst]

​DecimalField accepts max_digit parameter that specifies the amount of digit positions used by the DecimalField.

Consider the following model that allows for only one digit before the decimal separator:

from django.db.models import Model
class MyModel(Model):
    my_field = DecimalField(max_digits=2, decimal_places=1)

A value of Decimal("10.1") is not supported, as it requires 3 decimal places, however its input is accepted until the value is being inserted in the database.

​This line in django.db.backends.utils.format_number fails when any insertion or update is attempted (e.g. using save() , bulk_create etc. ), because quantize raises an InvalidOperation when there are not enough positions to fit the rounded value (according to Decimal.quantize ​docs).

The error message is not very informative (0).

​This commit contains a test that triggers the error. Note that overflow might also happen because of rounding during the quantization (e.g. values of 9.99999 could result in 10.0 during quantization, depending on the ​Context used).
For a copy of the commit see (1).

(0) Example error message

Traceback (most recent call last):
  File "/path_to_project/file.py", line 123, in my_failing_function
    overflowing_instance.save()
  File "/path_to_django/db/models/base.py", line 796, in save
    force_update=force_update, update_fields=update_fields)
  File "/path_to_django/db/models/base.py", line 824, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/path_to_django/db/models/base.py", line 908, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/path_to_django/db/models/base.py", line 947, in _do_insert
    using=using, raw=raw)
  File "/path_to_django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/path_to_django/db/models/query.py", line 1046, in _insert
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/path_to_django/db/models/sql/compiler.py", line 1053, in execute_sql
    for sql, params in self.as_sql():
  File "/path_to_django/db/models/sql/compiler.py", line 1006, in as_sql
    for obj in self.query.objs
  File "/path_to_django/db/models/sql/compiler.py", line 1006, in <listcomp>
    for obj in self.query.objs
  File "/path_to_django/db/models/sql/compiler.py", line 1005, in <listcomp>
    [self.prepare_value(field, self.pre_save_val(field, obj)) for field in fields]
  File "/path_to_django/db/models/sql/compiler.py", line 945, in prepare_value
    value = field.get_db_prep_save(value, connection=self.connection)
  File "/path_to_django/db/models/fields/__init__.py", line 1587, in get_db_prep_save
    return connection.ops.adapt_decimalfield_value(self.to_python(value), self.max_digits, self.decimal_places)
  File "/path_to_django/db/backends/base/operations.py", line 495, in adapt_decimalfield_value
    return utils.format_number(value, max_digits, decimal_places)
  File "/path_to_django/db/backends/utils.py", line 204, in format_number
    value = value.quantize(decimal.Decimal(".1") ** decimal_places, context=context)
decimal.InvalidOperation: [<class 'decimal.InvalidOperation'>]

(1)Test that triggers the error (add to django.tests.model_fields.test_decimalfield.DecimalFieldTests)

    def test_save_with_max_digits_overflow(self):
        """
        Ensure overflowing decimals yield a meaningful error.
        """
        overflowing_value = Decimal(10 ** 6)
        expected_message = "Not enough digit positions in field 'd' to represent {}".format(overflowing_value) # some meaningful error message
        overflowing_instance = Foo(a='a', d=overflowing_value)
        with self.assertRaisesMessage(ValidationError, # some meaningful error
            expected_message):
            overflowing_instance.save()

[Tim Graham]

If you want a nice error message, you need to run validation before calling save: overflowing_instance.full_clean().

Doing so in your test raises ValidationError: {'d': ['Ensure that there are no more than 5 digits in total.']}.