So often it is important to have a ForeignKey connection that starts with nothing and then has something added
here are my models
class Class(models.Model):
title = models.CharField(max_length=30)
threshold = models.PositiveIntegerField(validators=[MaxValueValidator
(100),MinValueValidator(60)])
works = models.ForeignKey(Work,null=True) #blank=True also doesn't work
so I create a class that has no Work, which logically makes sense:
math = Class("math",90)
math.save()
now lets say we have homework and quizzes that are Work instances:
math.works.add(homework,quizzes)
#gives the following error
AttributeError: 'NoneType' object has no attribute 'add'
math.works is clearly empty to start, and that makes sense, but this will not let me add anything!
How would I be able to start a ForeignKey with nothing and then be able to add to it?
My two cents. First, i think you should move your foreignkey to work class not in Class class. add() is for manytomany fieldSee documentation
class work(models.Model):
.....
subject = models.ForeignKey(Class,null=True)
So that the relationship is built as your intention. homework, quizzes as instances of work class both are related to math as an instance of Class class.
when you try to save, do as following:
>>> homework = work.objects.get(pk=1)
>>> math = Class.objects.get(pk=1)
>>> homework.subject = math
>>> homework.save()
>>> quizzes = work.objects.get(pk=2)
>>> math = Class.objects.get(pk=1)
>>> quizzes.subject = math
>>> quizzes.save()
Now you can retrieve all the work for given Class:
math = Class.objects.get(pk=1)
math.work_set.all()
it will return you all the instances that are related to math, and in your case, will be homework and quizzes.
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