多个 HTML 选项选择不起作用

我正在使用 django 框架开发任务管理系统，主管可以使用 Django 多对多字段登录并将任务分配给多个用户。当我登录管理门户时，我可以同时选择多个用户，这很好地保存到数据库中。但是当我使用前端模板时，我可以选择多个用户，但所选的选项永远不会保存在数据库中，而从数据库表中查看时，该字段将为空白。

这是我的模型：

from django.contrib.auth.models import User

class Task(models.Model):
    task_title = models.CharField(max_length=30, blank=True, null=True)
    unit = models.ForeignKey(Unit, blank=True, null=True)
    audit_phase_choice = (
            ('Pre Engagement', 'Pre Engagement'),
            ('Understanding Entity', 'Understanding Entity'),
            ('Risk Assessment', 'Risk Assessment'),
            ('Performing Audit', 'Performing Audit'),
            ('Report', 'Report'),
        )
    audit_phase = models.CharField(max_length=30, blank=True, null=True, choices=audit_phase_choice)

    assigned_by = models.CharField(max_length=30, blank=True, null=True)
    assigned_to = models.ManyToManyField(User, blank=True)
    date_assigned = models.DateTimeField(auto_now_add=False, auto_now=False, blank=True, null=True)
    status = models.CharField(max_length=30, blank=True, null=True)
    completed = models.BooleanField('Task Completed', default=False)
    date_completed = models.DateTimeField(auto_now_add=False, auto_now=False, blank=True, null=True)
    start_date = models.DateTimeField(auto_now_add=False, auto_now=False, blank=True, null=True)
    due_date = models.DateField(auto_now_add=False, auto_now=False, blank=True, null=True)
    comment = models.TextField('comments', max_length=3000, default='', blank=True, null=True)
    timestamp = models.DateTimeField(auto_now_add=True, auto_now=False, blank=True)

    def __unicode__(self):
       return self.task_title

    def get_absolute_url(self):
       return reverse("taskmis:user_task_edit", kwargs={"id": self.id})

这是form.py的内容

class TaskForm(forms.ModelForm):
    class Meta:
        model = Task
        fields = ['task_title',
                'unit',
                'assigned_to',
                'start_date',
                'due_date',
                'comment']

这是 view.py 的内容：

def user_task_entry(request):
    title = 'Assign Task'
    form = TaskForm(request.POST or None) 
    if form.is_valid():
        instance = form.save(commit=False)
        instance.assigned_by = request.user
        instance.save()
        return redirect('taskmis:user_task_list')
    context = {
        "title": title,
        "form": form,
     }
    return render(request, "task_entry.html",context)