[Ncr-Python.in] why does to_python gets called twice in this code ?

[Nandeep Mali]

Your question still has inconsistencies and lack of clarity. Have replied
on SO itself.


On Wed, Jan 9, 2013 at 4:07 PM, Vijay Shanker <deontics at gmail.com> wrote:

> hi
> i rephrased the question.thanks for the help extended so far.
>
> http://stackoverflow.com/questions/14214530/issues-in-example-custom-field-inheriting-foreignkey
> regards
> vijay shanker
>
>
> On Tue, Jan 8, 2013 at 10:36 PM, Gora Mohanty <gora at mimirtech.com> wrote:
>
>> On 8 January 2013 18:02, Vijay Shanker <deontics at gmail.com> wrote:
>> > hi
>> > I am struck trying to write a simple custom django field that inherits
>> > ForeignKey ..i posted the same at stackoverflow..
>> >
>> >
>> http://stackoverflow.com/questions/14214530/why-is-to-python-called-twice-in-this-custom-field-when-saving-an-object-insta
>> > any help or pointers are welcome.
>>
>> It is hard to definitively tell why to_python() is called twice,
>> without digging through the Django admin. code. My guess is
>> that:
>> * As you have set SubfieldBase metaclass, it is called once when
>>   the field is created.
>> * It is called again on form validation: "The to_python() method
>>   on a Field is the first step in every validation."
>>   https://docs.djangoproject.com/en/dev/ref/forms/validation/
>>
>> However, the problem probably lies in your code, which seems
>> a bit confused for the ProfiledUserField class:
>> * to_python() should be prepared to accept values of None, if
>>   null=True is allowed. Please see the note at the bottom of
>>   the section
>> https://docs.djangoproject.com/en/dev/howto/custom-model-fields/#converting-database-values-to-python-objects
>> * Your to_python() function seems to serve no purpose. For
>>    a ForeignKey field that you derive from, the returned value
>>    should be an integer, and not an instance of User as you
>>    seem to be assuming.. Thus, the to_python() function, as
>>    in the base class (deriving from django.db.models.Field),
>>    could simply be:
>>    def to_python(self, value):
>>        return value
>>    I.e., there is no need to override this function that I can see.
>> * Your __init__() function serves no purpose either: max_length
>>    is not a valid keyword argument for
>>    django.db.models.fields.related.ForeignKey
>>
>> I stopped reading at this point as it is increasingly seeming
>> like a XY problem: http://people.apache.org/~hossman/#xyproblem
>> People might better be able to help you if you would explain
>> what you are trying to achieve.
>>
>> Regards,
>> Gora
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