Question

There is a Django field type for storing values of MyType.

from django.core.exceptions import ValidationError
from django.db import models

class MyTypeField(models.Field):
    __metaclass__ = models.SubfieldBase

    def db_type(self, connection):
        return "text"

    def to_python(self, value):
        if isinstance(value, basestring):
            try:
                value = MyType.deserialize(value)
            except ParseError, e:
                raise ValidationError("Invalid format: "+str(e))
        assert isinstance(value, MyType)
        return value

    def get_prep_value(self, value):
        if not isinstance(value, MyType):
            raise ValidationError("Not MyType")
        return value.serialize()

I am trying to use fields of this type on an admin page of a model. Everything works nice if a user enters a valid value in the field. But if the entered value is invalid, the ValidationError is not caught (you get error 500, or a stack trace if debug is enabled)

Instead I want to see the form with a message “Invalid format” near the field (just like if you enter an invalid date or number). How to modify the field class to get validation errors in a correct place.

Answer 1

Quoting from http://docs.djangoproject.com/en/1.2/howto/custom-model-fields/#modelforms-and-custom-fields

If you use SubfieldBase, to_python()
will be called every time an instance
of the field is assigned a value. This
means that whenever a value may be
assigned to the field, you need to
ensure that it will be of the correct
datatype, or that you handle any
exceptions.

So a ValidationError will never be caught at this point. to_python is also called when populating a new instance of your model from the database, which is outside the form validation context.

Therefore, you must ensure that the
form field used to represent your
custom field performs whatever input
validation and data cleaning is
necessary to convert user-provided
form input into a
to_python()-compatible model field
value. This may require writing a
custom form field, and/or implementing
the formfield() method on your field
to return a form field class whose
to_python() returns the correct
datatype.

So you have to move your validation into a formfield.