正如评论中提到的，您可以简单地为此编写一个迁移。我认为下面的方法应该可以工作，虽然我还没有测试它。它使用queryset update API和F来避免循环

from __future__ import unicode_literals

from django.apps import apps
from django.db import migrations, models
from django.db.models import F

def copy_field(apps, schema):
    MyModel = apps.get_model('<your app>', 'MyModel')
    MyModel.objects.all().update(column_a=F('column_b'))

class Migration(migrations.Migration):
    dependencies = [
        ('<your app>', '<previous migration>'),
    ]

    operations = [
        migrations.RunPython(code=copy_field),
    ]