Question

在使用Django 1.8和Python 2.7的模型中，我们进行了一次迁移，该迁移意外将FloatField的默认值设置为空字符串（default =''），Django似乎将其解释为字节字符串：

class Migration(migrations.Migration):

    dependencies = [
        ('reo', '0030_merge'),
    ]

    operations = [
        migrations.CreateModel(
            name='ProfileModel',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
                ('pre_setup_scenario_seconds', models.FloatField(default=b'', null=True)),
            ],
        ),
    ]

升级到Django 2.2和Python 3.6之后，Django用manage.py migrate发出警告，指出存在未迁移的更改。将默认值从''更改为None并运行manage.py makemigrations后，我们得到：

operations = [
        migrations.AlterField(
            model_name='profilemodel',
            name='pre_setup_scenario_seconds',
            field=models.FloatField(default=None, null=True),
        ),
]

然后运行manage.py migrate将产生：

nlaws-> python manage.py migrate
Operations to perform:
  Apply all migrations: auth, contenttypes, django_celery_results, proforma, reo, resilience_stats, sessions, summary, tastypie
Running migrations:
  Applying reo.0050_auto_20191030_1738...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/base.py", line 323, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 234, in handle
    fake_initial=fake_initial,
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/migrations/executor.py", line 245, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 249, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 535, in alter_field
    old_db_params, new_db_params, strict)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/backends/postgresql/schema.py", line 124, in _alter_field
    new_db_params, strict,
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 648, in _alter_field
    old_default = self.effective_default(old_field)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 233, in effective_default
    return field.get_db_prep_save(self._effective_default(field), self.connection)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 789, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 784, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/Users/nlaws/.virtualenv/api/lib/python3.6/site-packages/django/db/models/fields/__init__.py", line 1739, in get_prep_value
    return float(value)
ValueError: could not convert string to float:

有人知道如何解决此问题吗？

它似乎与https://code.djangoproject.com/ticket/28431有关，{{3}}的字符串值设置为二进制字段的默认值-似乎是Django源代码中的错误。

Answer 1

return float(value) ValueError: could not convert string to float in Django models的建议有助于解决此问题，特别是我们最终得到了解决：

创建一个新的float字段2
运行管理命令以从一个字段中获取数据，将其转换为浮点数，然后将其保存到另一字段中
删除第一个（字符串）字段
将第二个字段（float字段）重命名为您需要的名称。

第2步是通过migrations.RunSQL和

完成的

UPDATE reo_profilemodel SET pre_setup_scenario_seconds2 = cast(pre_setup_scenario_seconds as double precision)

升级Django和Python版本，将FloatField的默认值设置为字符串的问题

1 个答案: