allow DecimalFields with max_digits == decimal_places

[Dan Watson]

This was discussed on django-dev here:

​https://groups.google.com/d/topic/django-developers/TvMNbPWwetY/discussion

Essentially, the fix for #7726 caused a validation error for max_digits == decimal_places, which is a valid numeric type in the database.

[Dan Watson]

Patch against trunk r16579, all tests pass.

[Matthias Kestenholz]

The documentation needs to be changed, too:

.. attribute:: DecimalField.max_digits

    The maximum number of digits allowed in the number. Note that this number
    must be greater than ``decimal_places``, if it exists.

[Dan Watson]

Updated with documentation patch

[Malcolm Tredinnick]

Feels like there should be a test included here. However, my reading of the original ticket we are modifying is that there was a problem with Django normalising .1234 to be 0.1234, so maybe a test is hard to write.

However, failing an argument that it's impossible to test at the Django level (and hence only relevant when Django creates the table), it needs a test to avoid future regressions.

[Dan Watson]

I think the problem was that Django used to do it's own validation of decimals (see #7064 and the associated patch). It seems now, Django simply tries the Decimal constructor and catches any exception. Since Decimal('0.123') == Decimal('.123') (both get normalized to include the leading 0) I don't think this is an issue.

[Jacob]

Milestone 1.4 deleted

[kenth]

Updated patch to account for refactoring in #16681

[Karen Tracey]

In [17089]:

Fix #16570: Restore ability to have decimal fields where max_digits equals decimal_places. Thanks dcwatson and kenth.