[Fixed]-Django: Validator for a multiple choice form field

My guess is that value, returned by CheckboxSelectMultiple‘s value_from_datadict method, is list [u'abc', u'xyz']. Then it is converted to string [u'abc', u'xyz'] by field’s to_python method (actually, it is u"[u'abc', u'xyz']"). After validators are run. Length of this string is more than 3, that’s why you got ValidationError.

You should use ManyToManyField.

The reason why my validator wasn’t working as intended was because the argument passed to the validator, say [u'abc', u'xyz'], was a string and not a list as I earlier thought while defining my validator. As a result, as rightly pointed out by @f43d65, the length of the argument exceeded 3 every single time, and hence the ValidationError was being raised.

I made the following changes in my validator in order for it to work as intended:

def len_area(li):
    if li.count("u'") > 3:
        raise ValidationError("Please select a maximum of three fields only")