django - 如何忽略特定的迁移？

Question

我有这样的迁移:

class Migration(migrations.Migration):

    dependencies = [
        ('app', '0020_auto_20191023_2245'),
    ]

    operations = [
        migrations.AddField(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    ]

但是它引发了一个错误:

django.db.utils.ProgrammingError: column "theme" of relation "app_agenda" already exists

没问题，我已经像这样包装了这个错误:

from django.db import migrations, models, ProgrammingError


def add_field_theme_to_agenda(apps, schema_editor):
    try:
        migrations.AddField(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    except ProgrammingError as e:  # sometimes it can exist
        if "already exists" not in str(e):
            raise


class Migration(migrations.Migration):
    dependencies = [
        ('app', '0020_auto_20191023_2245'),
    ]
    operations = [
        migrations.RunPython(add_field_theme_to_agenda),
    ]

这就像一个魅力，所有以下迁移都已完成。

我的问题是，每次我运行“makemigrations”时，Django 都会再次添加迁移(= 我的问题顶部的那个)。我猜这是因为它在迁移中看不到它，因为我的代码混淆了它。

如何使用迁移来避免这种情况(不要说“这个问题出在您的数据库上，请更正您的数据库”之类的答案)？

Answer 1

Django 正在重新创建迁移，因为当您在 RunPython 操作中手动执行操作时，它无法理解添加的字段。您可以尝试的是(我自己还没有尝试过)，将 AddField 操作子类化以创建自定义 AddField 操作，您可以在其中处理异常。像下面这样的东西可以工作:

from django.db import migrations, models, ProgrammingError


class AddFieldIfNotExists(migrations.AddField):

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        try:
            super().database_forwards(app_label, schema_editor, from_state,
                                      to_state)
        except ProgrammingError as e:  # sometimes it can exist
            if "already exists" not in str(e):
                raise


class Migration(migrations.Migration):
    atomic = False
    dependencies = [
        ('app', '0070_auto_20191023_1203'),
    ]

    operations = [
        AddFieldIfNotExists(
            model_name='agenda',
            name='theme',
            field=models.PositiveIntegerField(default=1),
        ),
    ]