Question

我正在尝试创建自定义字段。它基于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)

我创建了一个包含此字段的模型：

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__

我进行迁移没有问题，但是当我尝试迁移时，会发生这种情况：

(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'

get_prep_value()是否必须处理None？我已经阅读了docs，但事实并非如此，至少在他们的代码示例中并非如此。谁能解释一下这里出了什么问题？

Answer 1

感谢@solarissmoke提供此解决方案：

get_prep_value必须处理None，因为我已为该字段设置null=True：

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)

get_prep_value（）必须处理value = None吗？

1 个答案: