如何以编程方式为 Django 中的给定模型生成 CREATE TABLE SQL 语句?

Suppose you have an external DB table, that you decided to access as a Django model and therefore have described it as an unmanaged model (Meta: managed = False). Later you need to be able to create it in your code, e.g for some tests using your local DB. Obviously, Django doesn't make migrations for unmanaged models and therefore won't create it in your test DB. This can be solved using Django APIs without resorting to raw SQL - SchemaEditor. See a more complete example below, but as a short answer you would use it like this:

   from django.db import connections

   with connections['db_to_create_a_table_in'].schema_editor() as schema_editor:
        schema_editor.create_model(YourUnmanagedModelClass)

一个实际的例子:

# your_app/models/your_model.py

from django.db import models

class IntegrationView(models.Model):
    """A read-only model to access a view in some external DB."""

    class Meta:
        managed = False
        db_table = 'integration_view'

    name = models.CharField(
        db_column='object_name',
        max_length=255,
        primaty_key=True,
        verbose_name='Object Name',
    )
    some_value = models.CharField(
        db_column='some_object_value',
        max_length=255,
        blank=True,
        null=True,
        verbose_name='Some Object Value',
    )

    # Depending on the situation it might be a good idea to redefine
    # some methods as a NOOP as a safety-net.
    # Note, that it's not completely safe this way, but might help with some
    # silly mistakes in user code

    def save(self, *args, **kwargs):
        """Preventing data modification."""
        pass

    def delete(self, *args, **kwargs):
        """Preventing data deletion."""
        pass

现在，假设您需要能够通过 Django 创建此模型，例如进行一些测试.

Now, suppose you need to be able to create this model via Django, e.g. for some tests.

# your_app/tests/some_test.py

# This will allow to access the `SchemaEditor` for the DB
from django.db import connections
from django.test import TestCase
from your_app.models.your_model import IntegrationView

class SomeLogicTestCase(TestCase):
    """Tests some logic, that uses `IntegrationView`."""

    # Since it is assumed, that the `IntegrationView` is read-only for the
    # the case being described it's a good idea to put setup logic in class 
    # setup fixture, that will run only once for the whole test case
    @classmethod
    def setUpClass(cls):
        """Prepares `IntegrationView` mock data for the test case."""

        # This is the actual part, that will create the table in the DB
        # for the unmanaged model (Any model in fact, but managed models will
        # have their tables created already by the Django testing framework)
        # Note: Here we're able to choose which DB, defined in your settings,
        # will be used to create the table

        with connections['external_db'].schema_editor() as schema_editor:
            schema_editor.create_model(IntegrationView)

        # That's all you need, after the execution of this statements
        # a DB table for `IntegrationView` will be created in the DB
        # defined as `external_db`.

        # Now suppose we need to add some mock data...
        # Again, if we consider the table to be read-only, the data can be 
        # defined here, otherwise it's better to do it in `setUp()` method.

        # Remember `IntegrationView.save()` is overridden as a NOOP, so simple
        # calls to `IntegrationView.save()` or `IntegrationView.objects.create()`
        # won't do anything, so we need to "Improvise. Adapt. Overcome."

        # One way is to use the `save()` method of the base class,
        # but provide the instance of our class
        integration_view = IntegrationView(
            name='Biggus Dickus',
            some_value='Something really important.',
        )
        super(IntegrationView, integration_view).save(using='external_db')

        # Another one is to use the `bulk_create()`, which doesn't use
        # `save()` internally, and in fact is a better solution
        # if we're creating many records

        IntegrationView.objects.using('external_db').bulk_create([
            IntegrationView(
                name='Sillius Soddus',
                some_value='Something important',
            ),
            IntegrationView(
                name='Naughtius Maximus',
                some_value='Whatever',
            ),
        ])

    # Don't forget to clean after
    @classmethod
    def tearDownClass(cls):
        with connections['external_db'].schema_editor() as schema_editor:
            schema_editor.delete_model(IntegrationView)

    def test_some_logic_using_data_from_integration_view(self):
         self.assertTrue(IntegrationView.objects.using('external_db').filter(
             name='Biggus Dickus',
         ))

为了使示例更完整...由于我们使用了多个数据库(default 和 external_db)，Django 将尝试在这两个数据库上运行迁移测试，到目前为止，数据库设置中没有选项可以防止这种情况发生.所以我们必须使用自定义的 DB 路由器进行测试.

To make the example more complete... Since we're using multiple DB (default and external_db) Django will try to run migrations on both of them for the tests and as of now there's no option in DB settings to prevent this. So we have to use a custom DB router for testing.

 # your_app/tests/base.py

class PreventMigrationsDBRouter:
    """DB router to prevent migrations for specific DBs during tests."""
    _NO_MIGRATION_DBS = {'external_db', }

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """Actually disallows migrations for specific DBs."""
        return db not in self._NO_MIGRATION_DBS

以及所描述案例的测试设置文件示例:

And a test settings file example for the described case:

# settings/test.py

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.oracle',
        'NAME': 'db_name',
        'USER': 'username',
        'HOST': 'localhost',
        'PASSWORD': 'password',
        'PORT': '1521',
    },
    # For production here we would have settings to connect to the external DB,
    # but for testing purposes we could get by with an SQLite DB 
    'external_db': {
        'ENGINE': 'django.db.backends.sqlite3',
    },
}

# Not necessary to use a router in production config, since if the DB 
# is unspecified explicitly for some action Django will use the `default` DB
DATABASE_ROUTERS = ['your_app.tests.base.PreventMigrationsDBRouter', ]

希望这个详细的新 Django 用户友好示例能够帮助某人并节省他们的时间.

Hope this detailed new Django user user-friendly example will help someone and save their time.

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