How to define a Django model with a reference to itself?

Question

I'm new to Django and currently write an application, where users can enter their opinions on some topic.

Every opinion A may have zero or more opinions, which support A and several (zero or more) opinions, which refute A.

I tried to create a model for this and wrote a models.py file like this:

from django.db import models

# Create your models here.
class Opinion(models.Model):
    id = models.AutoField(primary_key=True)
    contents = models.CharField(max_length=256)
    source = models.CharField(max_length=256)
    proArguments = models.ManyToManyField(Opinion, verbose_name="Pro arguments")
    contraArguments = models.ManyToManyField(Opinion, verbose_name="Contra arguments")

When I run python manage sqlall, I get following error:

  File "D:\dev\history-site\history_site\opinions\models.py", line 4, in <module>
    class Opinion(models.Model):
  File "D:\dev\history-site\history_site\opinions\models.py", line 8, in Opinion
    proArguments = models.ManyToManyField(Opinion, verbose_name="Pro arguments")

NameError: name 'Opinion' is not defined

How can I fix this error?

Answer 1

From docs of ManyToManyField:

Requires a positional argument: the class to which the model is related. This works exactly the same as it does for ForeignKey, including all the options regarding recursive and lazy relationships.

Which says:

To create a recursive relationship -- an object that has a many-to-one relationship with itself -- use models.ForeignKey('self').

So:

proArguments = models.ManyToManyField("self", verbose_name="Pro arguments")
contraArguments = models.ManyToManyField("self", verbose_name="Contra arguments")

I'm slightly wondering about your data model if arguments are considered opinions as well, but that's another matter.

Answer 2

You would have to use self. And since, you have two m2m self relationship you need to add a related_name argument or provide symmetrical=False

class Opinion(models.Model):
    id = models.AutoField(primary_key=True)
    contents = models.CharField(max_length=256)
    source = models.CharField(max_length=256)
    proArguments = models.ManyToManyField('self', verbose_name="Pro arguments", related_name='my_proargs')
    contraArguments = models.ManyToManyField('self', verbose_name="Contra arguments", related_name='my_contraarg')

Answer 3

The way to do this is to use 'self'

proArguments = models.ManyToManyField('self', verbose_name="Pro arguments")
contraArguments = models.ManyToManyField('self', verbose_name="Contra arguments")

From django documentation:

To create a recursive relationship -- an object that has a many-to-one relationship with itself -- use models.ForeignKey('self').