title: Django "DecimalFields must define a 'decimal_places' attribute."
body: <p>One of my model fields is the following:</p>
  
  <pre><code>aaf_1kg_all = models.DecimalField(blank=True, null=True)
  </code></pre>
  
  <p>When I use my model normally, everything is fine. When I use it in a <a href="https://docs.djangoproject.com/es/1.9/ref/applications/#django.apps.AppConfig.ready" rel="noreferrer"><code>ready()</code> hook</a>, however, I get this error:</p>
  
  <pre><code>SystemCheckError: System check identified some issues:
  
  ERRORS:
  myapp.Model.aaf_1kg_all: (fields.E130) DecimalFields must define a 'decimal_places' attribute.
  myapp.Model.aaf_1kg_all: (fields.E132) DecimalFields must define a 'max_digits' attribute.
  </code></pre>
  
  <p>Django docs say that these two attributes are optional. I saw <a href="https://stackoverflow.com/questions/30960589/why-is-django-decimalfield-attribute-max-digits-non-optional">this answer</a>, but in the database both decimal places and max_digits are defined.</p>
  
  <p>If I decide to add these attributes, i.e.</p>
  
  <pre><code>aaf_1kg_all = models.DecimalField(blank=True, null=True, max_digits=10, decimal_places=10)
  </code></pre>
  
  <p>the app runs, but at some point I get this error instead:</p>
  
  <pre><code>Traceback (most recent call last):
  [...]
    variants.extend(list(sub_qs))   # sub_qs is a QuerySet
  File ".../django/db/models/query.py", line 258, in __iter__
    self._fetch_all()
  File ".../django/db/models/query.py", line 1074, in _fetch_all
    self._result_cache = list(self.iterator())
  File ".../django/db/models/query.py", line 68, in __iter__
    for row in compiler.results_iter(results):
  File ".../django/db/models/sql/compiler.py", line 808, in results_iter
    row = self.apply_converters(row, converters)
  File ".../django/db/models/sql/compiler.py", line 792, in apply_converters
    value = converter(value, expression, self.connection, self.query.context)
  File ".../django/db/backends/sqlite3/operations.py", line 233, in convert_decimalfield_value
    value = expression.output_field.format_number(value)
  File ".../django/db/models/fields/__init__.py", line 1608, in format_number
    return utils.format_number(value, self.max_digits, self.decimal_places)
  File ".../django/db/backends/utils.py", line 200, in format_number
    value = value.quantize(decimal.Decimal(".1") ** decimal_places, context=context)
  decimal.InvalidOperation: [&lt;class 'decimal.InvalidOperation'&gt;]
  </code></pre>
  
  <p>Did I forget something ?</p>