title: how to call back unique together constraints as a field django
body: <p>i'm trying to call back unique constraints field , in my project i have to count number of M2M selected</p>
  <pre><code>class Booking(models.Model):
      room_no = models.ForeignKey(Room,on_delete=models.CASCADE,blank=True,related_name='rooms')
      takes_by = models.ManyToManyField(Vistor)
  
      @property
      def no_persons(self):
          qnt =  Booking.objects.filter(takes_by__full_information=self).count()#but this doesnt  work
          return qnt
  </code></pre>
  <blockquote>
  <p>Cannot query &quot;some room information&quot;: Must be &quot;Vistor&quot; instance.</p>
  </blockquote>
  <pre><code>class Vistor(models.Model):
      full_name = models.CharField(max_length=150)
      dob = models.DateField(max_length=14)
      city = models.ForeignKey(City,on_delete=models.CASCADE)
  
      class Meta:
          constraints = [
              models.UniqueConstraint(fields=['full_name','dob','city'],name='full_information')
          ]
  
      def __str__(self):
          return f'{self.full_name} - {self.city} - {self.dob}'
  </code></pre>
  <p>it it possible to access <code>full_information</code> through <code>Booking</code> model ? thank you ..</p>