How to add value to a many-to-many field in Django

Question

Suppose I have a relation as down below:

class Student(AbstractBaseUser):
    # It inherits AbstractBaseUser and has it's own manger
    # note: I did forget to add editable here
    id = models.UUIDField(primary_key=True, default=uuid.uuid4)
    firstname
    lastname
    ...


class Teacher(models.Model):
    id = models.UUIDField(default=uuid.uuid4, primary_key=True)
    firstname
    lastname
    email
    ...


class Meeting(models.Model):
    id = models.UUIDField(default=uuid.uuid4, primary_key=True, editable=False)
    student = models.ManyToManyField(Student)
    teacher = models.ManyToManyField(Teacher)

How should I add/set a value to field 'student' or 'teacher'. I tried to assign an object from related models to them but got an error:

Direct assignment to the forward side of a many-to-many set is prohibited. Use student.set() instead.

And I tried this as well:

m = Meeting()
s = Student.objects.first() #to get a random student
m.student.add(s)

Then I get: ValueError: Cannot add "<Student: sarah@gmail.com>": instance is on database "None", value is on database "default"

I did this as well:

m = Meeting.objects.create() #It does only have two mentioned fields.
m.student.add(s)

I get this: django.db.utils.ProgrammingError: column "meeting_id" is of type bigint but expression is of type uuid LINE 1: ..._meeting" ("meeting_id", "student_id") VALUES ('c7688df9-... ^ HINT: You will need to rewrite or cast the expression.

Does this answer your question? [How to add multiple objects to ManyToMany relationship at once in Django ?](https://stackoverflow.com/questions/4959499/how-to-add-multiple-objects-to-manytomany-relationship-at-once-in-django) Better yet simply have a look at the docs: https://docs.djangoproject.com/en/3.2/topics/db/examples/many_to_many/ — Abdul Aziz Barkat, Jun 08 '21 at 18:14
@AbdulAzizBarkat I tried this an I got an error. let me update the topic. Thanks — The Mir, Jun 08 '21 at 18:17
@TheMir: you should first *create* the `Meeting` at the database side, so `m = Meeting.objects.create()`, or `m.save()`, otherwise it has no pk, and you thus can not link students/teachers to the `Meeting`. — Willem Van Onsem, Jun 08 '21 at 18:21
Does one (or more) of your models use an `uuid` as primary key instead of an `AutoField`? — Willem Van Onsem, Jun 08 '21 at 18:30

Willem Van Onsem · Accepted Answer · 2021-06-08T18:57:13.827

4

You create a Meeting and than populate it with Teachers and Students, so:

s1 = Student.objects.create(firstname='Foo', last_name='Bar')
s2 = Student.objects.create(firstname='Qux', last_name='Qux')
t1 = Teacher.objects.create(
    firstname='Some',
    last_name='One',
    email='someone@something.com'
)

m = Meeting.objects.create()
m.student.add(s1, s2)
m.teacher.add(t1)

For more information, see the Many-to-many relationships section of the documentation.

Based on the exception you shared in the question, you created the Meeting model before you made primary keys of type UUIDField. We can fix this problem by:

removing the Meeting model;
run makemigrations;
create a new Meeting model;
run makemigrations again; and
run migrate to update the database.

edited Jun 08 '21 at 18:57

answered Jun 08 '21 at 18:14

Willem Van Onsem

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Actually I tried this as well. I got an error. Let me add error to the topic. – The Mir Jun 08 '21 at 18:21
@TheMir: as specified in the comments, you need to *create* a database record for the `Meeting` first, so `m = Meeting()` is *not* sufficient, you need to `m.save()`, or simply use `Meeting.objects.create()`. – Willem Van Onsem Jun 08 '21 at 18:27
I did this. added the error to the topic. I think It has an error related to types in DB. Considering all the models have id with type uuid4. – The Mir Jun 08 '21 at 18:30
@TheMir: It might be better to *share* the details regarding the primary keys of your models. – Willem Van Onsem Jun 08 '21 at 18:31
My bad. Just a moment. – The Mir Jun 08 '21 at 18:32
I added more details. – The Mir Jun 08 '21 at 18:36
@TheMir: is it possible that you *first* constructed the `Meeting` model, and *then* changed the primary keys to a UUIDField? – Willem Van Onsem Jun 08 '21 at 18:43
Yeah Yeah. That's what I did. – The Mir Jun 08 '21 at 18:45
@TheMir: if there are no meetings yet, it might be better to remove the `Meeting` model, make migrations, and then add a new one, such that the `ForeignKey`s link to the correct field. – Willem Van Onsem Jun 08 '21 at 18:46
Yes. It did work. I just removed the Meeting model then did migrations. finally Added the Meeting model and migrated again. Would you like to add a new answer to approve it? – The Mir Jun 08 '21 at 18:53

score 0 · Answer 2 · answered Jun 08 '21 at 18:28

First save Student objects and Teacher objects

>> student_data = {'firstname': 'John', 'last_name', 'Doe', }
>> student1 = Student.objects.create(**student_data )
>>student_data = {'firstname': 'Test', 'last_name', 'Test', }
>> student2 = Student.objects.create(**student_data )

>> teacher_data  = {'firstname': 'TeacherJohn', 'last_name': 'TeacherDoe', 'email': 'john.doe@mail.com'}
>> teacher = Teacher.objects.create(**teacher_data )

Then create a Meeting object with the other fields( other than m2m). Then add/set the m2m objects.

meeting = Meeting.objects.create(**the_model_fields_as_dict)
meeting.student.add(student1, student2)
meeting.teacher.add(teacher )

How to add value to a many-to-many field in Django

2 Answers2