from django.db import migrations
 
class MoveDataToNewTable(migrations.Operation):
    atomic = True
    
    def __init__(self, model_name, field_name, batch_size=1000):
        self.model_name = model_name
        self.field_name = field_name
        self.batch_size = batch_size
    
    def state_forwards(self, app_label, state):
        pass
    
    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        model = state.get_model(app_label, self.model_name)
        field = model._meta.get_field(self.field_name)
        
        queryset = model.objects.filter(**{f'{self.field_name}__isnull': False})
        total = queryset.count()
        
        for offset in range(0, total, self.batch_size):
            batch = list(queryset[offset:offset + self.batch_size])
            for obj in batch:
                field.save(obj, obj)
            schema_editor.execute('SELECT 1')
    
    def describe(self):
        return f'Migrating {self.field_name} data in {self.model_name}'
 
class Migration(migrations.Migration):
    dependencies = [('myapp', '0001_initial')]
    
    operations = [
        migrations.AddField(
            model_name='product',
            name='cached_slug',
            field=models.SlugField(max_length=150, null=True),
        ),
        MoveDataToNewTable('product', 'cached_slug'),
        migrations.AlterField(
            model_name='product',
            name='cached_slug',
            field=models.SlugField(max_length=150),
        ),
    ]