Django ModelForm多个复选框不起作用

from django import forms
from .models import MyModel

class MyModelForm(forms.ModelForm):
    my_field = forms.MultipleChoiceField(widget=forms.CheckboxSelectMultiple, choices=CHOICES)
    # 其他字段的定义...
    
    class Meta:
        model = MyModel
        fields = '__all__'

from django.shortcuts import render, redirect
from .forms import MyModelForm
from .models import MyModel

def my_view(request):
    if request.method == 'POST':
        form = MyModelForm(request.POST)
        if form.is_valid():
            form.save()
            return redirect('success')
    else:
        instance = MyModel.objects.get(pk=1)  # 假设有一个实例对象
        form = MyModelForm(instance=instance)
    
    return render(request, 'my_template.html', {'form': form})

<form method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <button type="submit">提交</button>
</form>