[python] How to limit the maximum value of a numeric field in a Django model?

Django has various numeric fields available for use in models, e.g. DecimalField and PositiveIntegerField. Although the former can be restricted to the number of decimal places stored and the overall number of characters stored, is there any way to restrict it to storing only numbers within a certain range, e.g. 0.0-5.0 ?

Failing that, is there any way to restrict a PositiveIntegerField to only store, for instance, numbers up to 50?

Update: now that Bug 6845 has been closed, this StackOverflow question may be moot. - sampablokuper

You can use Django's built-in validators—

from django.db.models import IntegerField, Model
from django.core.validators import MaxValueValidator, MinValueValidator

class CoolModelBro(Model):
    limited_integer_field = IntegerField(
        default=1,
        validators=[
            MaxValueValidator(100),
            MinValueValidator(1)
        ]
     )

Edit: When working directly with the model, make sure to call the model full_clean method before saving the model in order to trigger the validators. This is not required when using ModelForm since the forms will do that automatically.

from django.db import models
from django.core.validators import MinValueValidator, MaxValueValidator

size = models.IntegerField(validators=[MinValueValidator(0),
                                       MaxValueValidator(5)])

There are two ways to do this. One is to use form validation to never let any number over 50 be entered by a user. Form validation docs.

If there is no user involved in the process, or you're not using a form to enter data, then you'll have to override the model's save method to throw an exception or limit the data going into the field.

Here is the best solution if you want some extra flexibility and don't want to change your model field. Just add this custom validator:

#Imports
from django.core.exceptions import ValidationError      

class validate_range_or_null(object):
    compare = lambda self, a, b, c: a > c or a < b
    clean = lambda self, x: x
    message = ('Ensure this value is between %(limit_min)s and %(limit_max)s (it is %(show_value)s).')
    code = 'limit_value'

    def __init__(self, limit_min, limit_max):
        self.limit_min = limit_min
        self.limit_max = limit_max

    def __call__(self, value):
        cleaned = self.clean(value)
        params = {'limit_min': self.limit_min, 'limit_max': self.limit_max, 'show_value': cleaned}
        if value:  # make it optional, remove it to make required, or make required on the model
            if self.compare(cleaned, self.limit_min, self.limit_max):
                raise ValidationError(self.message, code=self.code, params=params)

And it can be used as such:

class YourModel(models.Model):

    ....
    no_dependents = models.PositiveSmallIntegerField("How many dependants?", blank=True, null=True, default=0, validators=[validate_range_or_null(1,100)])

The two parameters are max and min, and it allows nulls. You can customize the validator if you like by getting rid of the marked if statement or change your field to be blank=False, null=False in the model. That will of course require a migration.

Note: I had to add the validator because Django does not validate the range on PositiveSmallIntegerField, instead it creates a smallint (in postgres) for this field and you get a DB error if the numeric specified is out of range.

Hope this helps :) More on Validators in Django.

PS. I based my answer on BaseValidator in django.core.validators, but everything is different except for the code.

I had this very same problem; here was my solution:

SCORE_CHOICES = zip( range(1,n), range(1,n) )
score = models.IntegerField(choices=SCORE_CHOICES, blank=True)