Does get_prep_value() have to handle value = None? - django

Question

I'm trying to create a custom field. It's based on postgres.JSONField.
class CardiosField(JSONField):
"""Field representing a models.Cardios object"""
def from_db_value(self, value, expression, connection):
if value is None:
return value
return parse_cardios(value)
def to_python(self, value):
if isinstance(value, models.Cardios):
return value
if value is None:
return value
return parse_cardios(value)
def get_prep_value(self, value):
cardios_pre_json = [serie_object.pre_json() for serie_object in value.series]
return json.dumps(cardios_pre_json)
I've created a model that has this field:
class Workout(models.Model):
datetime = models.DateTimeField()
user = models.ForeignKey(User, on_delete=models.CASCADE)
lifts = fields.LiftsField(null=True)
cardios = fields.CardiosField(null=True)
def __str__(self):
return str(self.datetime)+" "+self.user.email
__repr__ = __str__
I make migrations without a problem, but when I try to migrate, this happens:
(workout) Sahands-MBP:workout sahandzarrinkoub$ python manage.py migrate
Operations to perform:
Apply all migrations: admin, auth, contenttypes, sessions, workoutcal
Running migrations:
Applying workoutcal.0003_auto_20171231_2308...Traceback (most recent call last):
File "manage.py", line 22, in <module>
execute_from_command_line(sys.argv)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/__init__.py", line 364, in execute_from_command_line
utility.execute()
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/__init__.py", line 356, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
self.execute(*args, **cmd_options)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
output = self.handle(*args, **options)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
fake_initial=fake_initial,
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/migrations/executor.py", line 115, in migrate
state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/migrations/executor.py", line 145, in _migrate_all_forwards
state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
state = migration.apply(state, schema_editor)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 87, in database_forwards
field,
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 415, in add_field
definition, params = self.column_sql(model, field, include_default=True)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 155, in column_sql
default_value = self.effective_default(field)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 229, in effective_default
default = field.get_db_prep_save(default, self.connection)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 770, in get_db_prep_save
prepared=False)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 762, in get_db_prep_value
value = self.get_prep_value(value)
File "/Users/sahandzarrinkoub/Documents/Programming/Web/Django/workout/workout/workoutcal/fields.py", line 52, in get_prep_value
cardios_pre_json = [serie_object.pre_json() for serie_object in value.series]
AttributeError: 'NoneType' object has no attribute 'series'
Does get_prep_value() have to deal with None? I've read the docs and this doesn't seem to be the case, at least not in their code example. Could anyone explain what's going wrong here?

Answer 1

Thank you #solarissmoke for this solution:
get_prep_value has to deal with None since I've set null=True for the field:
def get_prep_value(self, value):
if not value:
return value
cardios_pre_json = [serie_object.pre_json() for serie_object in value.series]
return json.dumps(cardios_pre_json)