Django Models - Conditionally adding an object to a ManyToManyField before saving

Question

Hello

The following does not quite work, its purpose is:

Upon save, check for the existence of 'supervisor' in the 'operators', and add it too them if not.

class JobRecord(models.Model):
    """JobRecord model"""

    project             = models.ForeignKey(Project)
    date                = models.DateField()
    supervisor          = models.ForeignKey(User, related_name='supervisor_set')

    operators           = models.ManyToManyField(User, related_name='operators_set', help_text='Include the supervisor here also.')

    vehicles            = models.ManyToManyField(Vehicle, blank=True, null=True)

    def __unicode__(self):
        return u"%s - %s" % (self.project.name,  self.date.strftime('%b %d'))

    # --- over ride methods ---- #

    def save(self, **kwargs):
        # this must be done to get a pk
        super(JobRecord, self).save(**kwargs)

        # which makes this comparison possible
        if self.supervisor not in self.operators.__dict__:
            self.operators.add(self.supervisor)

        # it seems to get this far ok, but alas, the second save attempt
        # does not seem to work!
        print self.operators.values()
        super(JobRecord, self).save(**kwargs)

Thanks for your expertise, would be 'expert'!

Answer 1

You can do something like this to check if the supervisor is in the operators:

if self.operators.filter(id=self.supervisor.id).count() == 0:

And you don't need to save a second time after modifying the many to many field. (Many to many relations are stored in their own table.)

Answer 2

Ok, I've modified the to make the following. Actually, either conditional seems to do the trick. The issue now is that the add() method is not working for me.

#...

def save(self, **kwargs):
    super(JobRecord, self).save(**kwargs)

    if self.operators.filter(id=self.supervisor.id).count() == 0:
    #if self.supervisor not in self.operators.values():

        # either conditional will get to this point
        self.operators.add(self.supervisor) # <-- this line doesn't save proper?

Answer 3

i have the same issue. if you are using a django form, do your check after the form is saved, and then add the many to many there. that was the only way i could get around it.