[Fixed]-Instantiate model instance with manytomany field in Django

by

15👍

✅

You don’t need to call save after the many2many.add()

You could also shorten the code to 2 lines:

flst = FollowerList.objects.create(follower=user)
flst.followed.add(user)
👤Yuji 'Tomita' Tomita

4👍

Yuji’s answer is correct. You can not add an object to a M2M field until it has been saved. I wanted to mention a shorter way to create instances though.

user = User.objects.get(username=uname)
flst = FollowerList(follower=user) #use key word args to assign fields
flst.save()
flst.followed.add(user)
# don't need to save after adding an object to many to many field.

I find that syntax slightly nicer than creating an empty instance and assigning fields. Though the objects.create() method (mentioned by Yuki) is nicer still.

👤Josh Smeaton

2👍

A late answer to this: you could also override the constructor (__init__) as follows:

class FollowerList(models.Model):
    follower = models.ForeignKey(User,related_name="follower")
    followed = models.ManyToManyField(User,related_name="followed"

    def __init__(*args, followed=[], **kwargs):
        super(FollowerList, self).__init__(*args, **kwargs)
        self.save()
        for user in followed:
            self.followed.add(user)

ie here I’ve explicitly handled the followed keyword argument in the __init__ function, while passing all other args and kwargs on to the default constructor.
The call to save makes sure that the object has been registered and can thus be used in an m2m relationship.

This then allows you to do create FollowerList with one line, eg 

flst = FollowerList(follower=user, followed=[user,])

Alternatively, as pointed out by Johannes, saving a model in the __init__ is not expected. The preferred approach would be to create a Manager method – see here for details: https://docs.djangoproject.com/en/1.9/topics/db/managers/
and then to create a FollowerList:

fl = FollowerList.objects.create(*args, followed, **kwargs)
👤jmetz

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