在Django模型中创建一个可调用函数来为属性生成值

Question

我有一个模型a，它有一个models.PositiveIntegerField()值，我需要使用从另一个相关模型B获得的一些信息来生成这个属性的值

class A(models.Model):
    ....
    num = models.PositiveIntegerField()
    ....

def get_val(instance):
    return instance.a.num + 1

Class B(models.Model):
    ...
    a   = models.ForeignKey(A, on_delete=models.CASCADE)
    val = models.PositiveIntegerField(default=get_val)
    ...

但是上面提到的方法不起作用，在执行python manage.py makemigrations之后尝试执行python manage.py migrate时出现了这个错误

TypeError: get_val() missing 1 required positional argument: 'instance'

详细错误：

Applying problemsetting.0011_testcase_index...Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/core/management/__init__.py", line 371, in execute_from_command_line
    utility.execute()
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/core/management/__init__.py", line 365, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/core/management/base.py", line 335, in execute
    output = self.handle(*args, **options)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 200, in handle
    fake_initial=fake_initial,
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/migrations/migration.py", line 122, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 84, in database_forwards
    field,
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/backends/sqlite3/schema.py", line 306, in add_field
    self._remake_table(model, create_field=field)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/backends/sqlite3/schema.py", line 178, in _remake_table
    self.effective_default(create_field)
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 218, in effective_default
    default = field.get_default()
  File "/mnt/ebram96/workspace/linux/django_projects/env3/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 775, in get_default
    return self._get_default()
TypeError: get_val() missing 1 required positional argument: 'instance'

我在虚拟环境下使用Python3和django 2。。你知道吗

Answer 1

如果要通过实例获取值，可以重写save方法：

Class B(models.Model):
    ...
    a   = models.ForeignKey(A, on_delete=models.CASCADE)
    val = models.PositiveIntegerField()
    #                               ^^^

    def save(self, *args, **kwargs):
        if not self.val:
            self.val = self.a.num + 1
        return super().save(*args, **kwargs)

更多详细信息，您可以阅读overriding-predefined-model-methods