强制到Unicode的Django:需要字符串或缓冲区，datetime。发现日期

I have a model:

我有一个模型:

class MyModel(models.Model):
    id = models.IntegerField(primary_key=True)
    recorded_on = models.DateField()
    precipitation = models.FloatField(null=True, blank=True)
class MyModel(model