Range Fields do not support blank values via ModelForm

[James Addison]

The filing of this issue is based on a discussion in IRC (see ​https://botbot.me/freenode/django/2018-08-09/?msg=103129716&page=12 and prior messages), followed up by creating a test project to reproduce the issue.

Please do correct me if I am misusing range fields.

---

Saving a modelform with a model's rangefield 2 inputs left empty triggers an DB integrity error. I think the culprit lies with empty_values not containing ['', ''] as a possible empty value. (see the code around ​https://github.com/django/django/blob/1.11.15/django/forms/fields.py#L1026).

self.compress([]) should return the result of self.range_type(None, None) instead of just None.

With a view like:

def home(request):
    if request.method == 'POST':
        form = RangeTestForm(request.POST)
        if form.is_valid():
            instance = form.save()
    else:
        form = RangeTestForm(request.POST)

    return render(request, 'rangefieldtest/home.html', {'form': form})

Form like:

class RangeTestForm(forms.ModelForm):
    class Meta:
        model = RangeTest
        fields = '__all__'

and Model like:

from django.contrib.postgres.fields import FloatRangeField
from psycopg2._range import NumericRange

class RangeTest(models.Model):
    name = models.CharField(max_length=50, blank=True, default='')
    age_range = FloatRangeField(blank=True, default=NumericRange)

I will attach a sample project demonstrating this (use runserver, load the home page, click save)

---

psql table definition:

                                  Table "public.rangefieldtest_rangetest"
  Column   |         Type          |                               Modifiers
-----------+-----------------------+-----------------------------------------------------------------------
 id        | integer               | not null default nextval('rangefieldtest_rangetest_id_seq'::regclass)
 name      | character varying(50) | not null
 age_range | numrange              | not null
Indexes:
    "rangefieldtest_rangetest_pkey" PRIMARY KEY, btree (id)

data successfully stored on save (when at least one of the rangefield's 2 inputs are filled in):

 id | name | age_range
----+------+-----------
  2 |      | [3.0,)
(1 row)

data successfully stored (see id=3 below) on RangeTest().save(), which should also be the result when saving from the modelform:

test_rangefield=> select * from rangefieldtest_rangetest;
 id | name | age_range
----+------+-----------
  2 |      | [3.0,)
  3 |      | (,)
(2 rows)

[James Addison]

sample project reproducing the issue