Allow only positive decimal numbers

You could do something like this :

# .....
class priceForm(ModelForm):
    price = forms.DecimalField(required=False, max_digits=6, min_value=0)

This, also, is responsible for the validator value of ‘price’.

According to the docs, it seems that there’s no way to put something like a database constraint on a field. The best you can do is add model “validators” which will be called if you call model validation or use a ModelForm. The validators are skipped if you simply put values into an object and save().

So, you can add validation on the forms or you can add validation to the model which also get run on the form level if you use ModelForm.

From the docs on “How validators are run”:

See the form validation for more information on how validators are run
in forms, and Validating objects for how they’re run in models. Note
that validators will not be run automatically when you save a model,
but if you are using a ModelForm, it will run your validators on any
fields that are included in your form. See the ModelForm documentation
for information on how model validation interacts with forms.

In Django 2.2 you can add constraints to a model which will be applied in the migrations as a constraint on the database table:

from decimal   import Decimal
from django.db import models

class Item(models.Model):
    price = models.DecimalField( _(u'Price'), decimal_places=2, max_digits=12 )

    class Meta:
        constraints = [
            models.CheckConstraint(check=models.Q(price__gt=Decimal('0')), name='price_gt_0'),
        ]

Note:

Validation of Constraints

In general constraints are not checked during full_clean(), and do not raise ValidationErrors. Rather you’ll get a database integrity error on save().

Assuming this is your product model, and you want to add non-negative constraint on price field. You can add the meta constraint on the model:

class Product(models.Model):
    price = models.DecimalField(max_digits=13, decimal_places=2)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        managed = True
        db_table = 'product'
        constraints = [
            models.CheckConstraint(check=models.Q(price__gte='0'), name='product_price_non_negative'),
        ]

1. Import dependencies

from decimal import Decimal
from django.core.validators import MinValueValidator

2. Add validators

price = models.DecimalField(
 decimal_places=2, 
 max_digits=12, 
 validators=[MinValueValidator(Decimal('0.01'))]
)