[Django] #29173: Models with custom fields returns the given value instead of stored value

[Django]

#29173: Models with custom fields returns the given value instead of stored 
value
-------------------------------------+-------------------------------------
               Reporter:  Xtreak     |          Owner:  nobody
                   Type:  Bug        |         Status:  new
              Component:  Database   |        Version:  2.0
  layer (models, ORM)                |
               Severity:  Normal     |       Keywords:
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
-------------------------------------+-------------------------------------
 When there is a custom field in the model and we do some transformation
 before storing it to the database. Hence while creating the object we give
 a value which is transformed and stored. But the returned object as a
 result of the create call has the given value instead of the transformed
 value.

 In the below model I have a custom field FooField which always returns
 "foo" as the value to be stored but when I create an object with another
 value say 'a' it only has the value 'a' stored in the returned object. I
 need to do a refresh_from_db to clear the wrong value and to fetch the
 correct value. I couldn't find this documented at
 [https://docs.djangoproject.com/en/2.0/howto/custom-model-fields/]. If
 this an expected behavior adding a note in the documentation will be of
 help.

 models.py

 {{{
 from __future__ import unicode_literals

 from django.db import models

 # Create your models here.

 class FooField(models.CharField):

     def to_python(self, value):
         value = super(self.__class__, self).to_python(value)
         return "foo"

     def get_prep_value(self, value):
         value = super(self.__class__, self).get_prep_value(value)
         return "foo"

 class FooBase(models.Model):

     base_field = models.CharField(max_length=120)

 class Foo(FooBase):

     custom_field = FooField(max_length=120)

 }}}

 Shell session

 {{{
 In [11]: bar = Foo.objects.create(base_field='1', custom_field='a')

 In [12]: bar.custom_field
 Out[12]: 'a'

 In [13]: bar.refresh_from_db()

 In [14]: bar.custom_field
 Out[14]: 'foo'

 In [15]: bar.id
 Out[15]: 10
 }}}

 {{{
 sqlite> select * from TestApp_foo;
 10|foo
 }}}

-- 
Ticket URL: <https://code.djangoproject.com/ticket/29173>
Django <https://code.djangoproject.com/>
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