我如何自动填充django中的字段？

from django.db import models

class MyModel(models.Model):
    my_field = models.CharField(max_length=100, default='default_value')

from django.db import models

class MyModel(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

from django.db import models
from django.dispatch import receiver
from django.db.models.signals import pre_save

class MyModel(models.Model):
    my_field = models.CharField(max_length=100)

@receiver(pre_save, sender=MyModel)
def set_my_field(sender, instance, **kwargs):
    instance.my_field = 'default_value'

from django import forms
from .models import MyModel

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ['my_field']

    def __init__(self, *args, **kwargs):
        super(MyModelForm, self).__init__(*args, **kwargs)
        self.fields['my_field'].initial = 'default_value'