多对多字段不会保存在主要对象中

如何解决多对多字段不会保存在主要对象中

我有一个对象：与另一个对象有1个许多关系的客户：全科医生。 我通过管理页面创建了一些常规医生实例。当我从addcustomer表单中现有gp的选择下拉列表中选择一个gp并提交我的表单时，该表单将与除所选gp之外的所有信息一起保存。我只有这个：app.GeneralPractitioner.None
这是我不同的班级

models.py

class Customer(models.Model):
   first_name = models.CharField(max_length=100)
   last_name = models.CharField(max_length=100)
   gp = models.ManyToManyField('GeneralPractitioner',help_text="Select your general practitioner",related_name="custome")

class GeneralPractitioner(models.Model):
   name = models.CharField(
   max_length=40,help_text="Enter the gp name)"
   )
   contact1 = models.CharField(max_length=11,blank=True)

   def __str__(self):
    return self.name

views.py

@login_required
def add_customer(request):
   if request.method == 'POST':

       form = AddCustomerForm(request.POST)
       if form.is_valid():
          instance = form.save(commit=False)
          instance.author = request.user
          instance.save()

          messages.success(request,f'Your form has been created!')
          return redirect('addcustomer')
   else:
      form = AddCustomerForm()
   return render(request,'app/addcustomer.html',{'form': form})

customer_details.html

{% extends "app/base.html" %}
{% load crispy_forms_tags %}
{% block content %}
<div>
    <p>Firstname: {{customer.first_name}}</p>
    <p>Lastname: {{customer.last_name}}</p>
    <p>email: {{customer.email}}</p>
    <p>General practitioner: {{customer.gp}}</p>
</div>