使用Django multiselectfield时出错

Question

我正在使用Django multiselectfield程序包，并且遇到了问题

ValueError: not enough values to unpack (expected 2, got 1)

我知道我在MultiSelectField属性choices中遇到问题，但是我不知道该怎么解决。

models.py

class Meal(models.Model):
    restaurant = models.ForeignKey(Restaurant, on_delete=models.PROTECT)
    category = models.ForeignKey(MealCategory, on_delete=models.PROTECT)
    name = models.CharField(max_length=500)
    short_description = models.CharField(max_length=500)
    image = models.ImageField(upload_to='meal_images/', blank=False)
    price = models.IntegerField(default=0)
#extras = models.OneToOneField('foodtaskerapp.Extra',related_name='is_extra' ,blank=True, null=True)
    extras = MultiSelectField(choices ='foodtaskerapp.Extra().get_extras()')
def __str__(self):
    return self.name


class Extra(models.Model):
    extra_n = models.CharField(max_length=200)
    meal = models.ForeignKey(Meal, on_delete = models.CASCADE, null=True, blank=True)
    extra_price = models.IntegerField(default=0)


    def __str__(self):
        return self.extra_n

    def get_extras(self):
         return self.extra_n