title: M2M relation Django, with custom intermediate table having an extra field
body: <p>I have two models like CinicHospital and Doctor with M2M relationship. The custom intermediate table is created because I need an extra field.</p>
  
  <pre><code>class DoctorHospital(models.Model):
  clinic = models.ForeignKey(ClinicHospital, on_delete=models.CASCADE)
  doctor = models.ForeignKey(Doctor, on_delete=models.CASCADE)
  shift = models.CharField(max_length = 10)
  </code></pre>
  
  <p><strong>Problem</strong> in this DoctorHospital model two fields are auto-filled while creating ClinicHospital and Doctor but I'm trouble to fill the shift field.
  Could I combine this shift field in doctor form and save it in intermediate table?</p>