Django : m2m relationship create two line instead of one - django

Question

I have extended the UserModel this way :
# users/models.py
from django.contrib.auth.models import AbstractUser
from django.db import models
class CustomUser(AbstractUser):
# add additional fields in here
credit = models.IntegerField(default=200)
follow = models.ManyToManyField('self', related_name='follow')
def __str__(self):
return self.username
But I am stuck as to how I should add/remove a follower. I have created a view with :
#login_required
def follow(request, user_id):
user = get_object_or_404(CustomUser, pk=user_id)
if CustomUser.objects.filter(follow=user.pk).exists():
request.user.follow.remove(user)
else:
request.user.follow.add(user)
return redirect('profil', user_id)
Issue :
Let's say request.user.pk is 1 and user_id is 2.
For the add part (in the else), I would expect a new line in database with from_customuser_id=1 and to_customuser_id=2 however, it creates two lines:
one with from_customuser_id=1 and from_customuser_id=2 as expected
one with from_customuser_id=2 and from_customuser_id=1 which I don't need.
And for the remove part (in the if), I would expect it to only remove the line
from_customuser_id=1 and from_customuser_id=2
But it removes the two lines.
I read the doc about django models relations but didn't found how to solve this issue.
Question :
How should I update my code in order to have the add method to only insert one line with from_customuser_id=1, from_customuser_id=2 and the remove method to only delete this line (assuming the current user have the id 1).
Not sure if it is relevant but for sake of completeness this is the related part of my urls.py :
path('follow/<int:user_id>', views.follow, name='follow'),
path('unfollow/<int:user_id>', views.follow, name='unfollow'),
And this is how I call them in templates :
{% if follow %}
<a href="{% url 'follow' user_profil.id %}">
Unfollow {{ user_profil.username }}
</a>
{% else %}
<a href="{% url 'unfollow' user_profil.id %}">
Follow {{ user_profil.username }}
</a>
{% endif %}

Answer 1

When you have a ManyToManyField it essentially creates a relationship between both the objects. This also allows you to do reverse lookups.
For example:
class Person(models.Model):
name = model.CharField(max_length=100)
class Pet(models.Model):
owners = models.ManyToMany(Person, related_name="pets")
name = model.CharField(max_length=100)
bob = Person.objects.create(name="Bob")
john = Person.objects.create(name="John")
kitty_kat = Pet.objects.create(name="Kitty Kat")
kitty_kat.owners.set([bob, john])
According to these models one pet can be owned by multiple people, and one person can have multiple pets. So if I do
bob.pets.all() # I get kitty kat
kitty_kay.owners.all() # I get bob & john
When this relationship is supposed to be on the same model, you end up creating two relationships. One as the normal one & one for the reverse.
For example:
class Person(models.Model):
name = model.CharField(max_length=100)
followers = models.ManyToManyField('self', related_name='follow')
bob = Person.objects.create(name="Bob")
john = Person.objects.create(name="John")
john.followers.add(bob)
bob.follow.all() # I get john... notice I use follow and not followers
john.followers.all() # I get bob
In order to avoid this you can pass symmetrical=False to the field and one one row will be created
followers = models.ManyToManyField('self', related_name='+', symmetrical=False)
Setting the related_name to anything starting with + will also prevent reverse lookups (which you don't need in this case)