Convert many to many relationship to one to many in django

What I'm trying to achieve

Till now we've models which have many to many relationships like given below.

class A(models.Model):
    name = models.CharField(max_length=255)
    data = models.ManyToManyField(B, related_name='data', through='C')

class B(models.Model):
    name=models.CharField(max_length=255)

class C(models.Model):
    a = ForeignKey(A)
    b = ForeignKey(B)
    c = model.CharField(max_length=255)
    active = models.BooleanField(default=True)
    and so on....

Although Django creates an intermediate table automatically for many to many relationships if we don't define it explicitly as you can notice I've defined the third intermediate table explicitly to store extra metadata about each relation.

So, as per new business requirements, we need to remove many to many relationships from the table and replace it with one to many relationships i.e each record of B can have more than one references to obj A but record of A will always have one reference to A obj.

also, since these tables are already being used in production therefore we need to migrate the existing data as well.

Things which I've tried so far

Removed many to many from model A.

class A(models.Model):
    name = models.CharField(max_length=255) 
    data = models.ForeignKey(B)

Updated the existing row of model C.

for eg:

#this might return more than one obj because of many to many relationship
test = C.objects.filter(a__id=<a obj id>)

for t in test:
   t.b_id = <some id pointing to record of B>

basically, after doing the above-mentioned steps, it will update the records with a_id to id of some obj of b.

Problem I'm Facing

when I tried to run makemigrations it asked for the default value of data, I ain't sure what to put there. In model A, I can point to the data field to any record of B, but then I'll lose the use of 3rd table C, also table C is being used widely across the system.
So, Do I need to drop table C?
also, wanted to know is there any better way to change many-to-many to one to many relationships.
Any help would be useful.

In such case when your data is inside an intermediary table,
I would recommend you to do the following.

1. Add and temporary column to your A table

class A(models.Model):
    name = models.CharField(max_length=255)
    data = models.ManyToManyField(B, related_name='data', through='C')
    # the on_delete part is not related, I added it just to avoid errors.
    tmp_data = models.ForeignKey(B, on_delete=models.DO_NOTHING)

and then do python manage.py makemigrations

2. Create an empty migration file and make it run this code to copy all A-B relation data from table C to table A

# Command
python manage.py makemigrations <<<APP_NAME>>> --empty

# Inside Empty migrations file
def migrate_c_to_a(apps, schema_editor):
    C = apps.get_model('<<<APP NAME>>>', 'C')
    for c in C.objects.all().iterator():
        a = c.a
        a.tmp_data = c.b
        a.save()

class Migration(migrations.Migration):
    ...
    ...
    operations = [
        migrations.RunPython(migrate_c_to_a)
    ]

3. Remove data column from you A model and makemigtations
4. Rename tmp_data column to data on your A model, then makemigrations
   > Careful, make sure that makemigrations won't remove the tmp_data and data and then adds new field data, this is going to cause loss of data, I will show you how to do it in the snippet below.

Inside migrations file, if this happened

...
operations = [
        migrations.RemoveField(
            model_name='a',
            name='tmp_data',
        ),
        migrations.RemoveField(
            model_name='a',
            name='data',
        ),
        migrations.AddField(
            model_name='a',
            name='data',
            field=models.ForeignKey(null=True, on_delete=django.db.models.deletion.DO_NOTHING, to='tsting.B'),
        ),
    ]
...

Change it to this

...
operations = [
        migrations.RemoveField(
            model_name='a',
            name='data',
        ),
        migrations.RenameField(
            model_name='a',
            old_name='tmp_data',
            new_name='data'
        ),
    ]
...

5. python manage.py migrate

This should do it for good.
after that, if you want to remove table C, it is up to you, since the data inside of it is safely migrated to table A