Django: Return 'None' from OneToOneField if related object doesn't exist?

Question

I\'ve got a Django class like this:

class Breakfast(m.Model):
    # egg = m.OneToOneField(Egg)
    ...

class Egg(m.Model):
    breakfast = m.OneToOneField(Break         


        

Answer 1

OmerGertel did already point out the null option. However, if I understand your logical model right, then what you actually need is a unique and nullable foreign key from Breakfast to Egg. So a breakfast may or may not have an egg, and a particular egg can only be associated with one breakfast.

I used this model:

class Egg(models.Model):
    quality = models.CharField(max_length=50)
    def __unicode__(self):
        return self.quality

class Breakfast(models.Model):
    dish = models.TextField()
    egg = models.ForeignKey(Egg, unique=True, null=True, blank=True)
    def __unicode__(self):
        return self.dish[:30]

and this admin definition:

class EggAdmin(admin.ModelAdmin):
    pass

class BreakfastAdmin(admin.ModelAdmin):
    pass

admin.site.register(Egg, EggAdmin)
admin.site.register(Breakfast, BreakfastAdmin)

Then I could create and assign an egg in the edit page for a breakfast, or just do not assign one. In the latter case, the egg property of the breakfast was None. A particular egg already assigned to some breakfast could not be selected for another one.

EDIT:

As OmerGertel already said in his comment, you could alternatively write this:

    egg = models.OneToOneField(Egg, null=True, blank=True)