Tukukkk

I'm trying to create a unique Availability status for Player on given Hour. Here goes the code:

class Player(models.Model):

  first_name = models.CharField(max_length=200)

  last_name = models.CharField(max_length=200)



class Hour(models.Model):

  date = models.DateTimeField()

  players = models.ManyToManyField(Player, blank=True,

                                   through='Availability')



class Availability(models.Model):

  player = models.ForeignKey(Player, on_delete=models.CASCADE)

  hour = models.ForeignKey(Hour, on_delete=models.CASCADE)

  available = models.BooleanField()

My problem is that it's currently possible to add same Availability few times - I want to programatically limit it to just one per combination.

Thanks in advance!

You can have the unique_together on the "throuh" table (in your case the Avaliability Table)

class Availability(models.Model):

  player = models.ForeignKey(Player, on_delete=models.CASCADE)

  hour = models.ForeignKey(Hour, on_delete=models.CASCADE)

  available = models.BooleanField()  

  class Meta:

    unique_together = (('player','hour'),)

And then use the get_or_create for adding/modifying:

 obj,created = Availability.objects.get_or_create(hour=hour,player=player)

I've worked around it modifying save() for Availability:

def save(self, *args, **kwargs):

    if self.pk is None:

        combination = Availability.objects.filter(player=self.player,

                                                  hour=self.hour).first()

        if combination:

            self.pk = combination.pk

    super(Availability, self).save(*args, **kwargs)

You can't have a unique constraint in a ManyToManyField, as reasoned in ticket 702. What you can do is validate the model when saving, as shown in an online snippet:

class MyModel(models.Model):

    site = models.ManyToManyField(Site)

    on_site = CurrentSiteManager()



    foobar = ... # A unique field



    def save(self, *args, **kwargs):

        if self.id == None: # new item should be created.

            # manually check a unique togeher, because django can't do this with a M2M field.

            # Obsolete if unique_together work with ManyToMany: http://code.djangoproject.com/ticket/702

            exist = MyModel.on_site.filter(foobar=self.foobar).count()

            if exist != 0:

                from django.db import IntegrityError

                # We can use attributes from this model instance, because it needs to have a primary key

                # value before a many-to-many relationship can be used.

                site = Site.objects.get_current()

                raise IntegrityError(

                    "MyModel item with same foobar field exist on site %r" % site

                )



        return super(MyModel, self).save(*args, **kwargs)



    class Meta:

        #unique_together = ("foobar", "site") # unique_together doesn't work with ManyToMany!

        # See: http://code.djangoproject.com/ticket/702

Note that the logic can change depending on your goal. As Carl Meyer argues in the ticket linked above, different behavior can make sense in your model (e.g. a field being unique with every item of the ManyToManyField or with the exact combination).