django - When is it required to use `schema_editor.connection.alias` in a django `migrations.RunPython` method?

Question

When is it required to use schema_editor.connection.alias in a migrations.RunPython method and why?

When using RunPython in a data migration, django advises that you should use schema_editor.connection.alias:

from django.db import migrations

def forwards_func(apps, schema_editor):
    # We get the model from the versioned app registry;
    # if we directly import it, it'll be the wrong version
    Country = apps.get_model("myapp", "Country")
    db_alias = schema_editor.connection.alias
    Country.objects.using(db_alias).bulk_create([
        Country(name="USA", code="us"),
        Country(name="France", code="fr"),
    ])

class Migration(migrations.Migration):

    dependencies = []

    operations = [
        migrations.RunPython(forwards_func, reverse_func),
    ]

Warning

RunPython does not magically alter the connection of the models for you; any model methods you call will go to the default database unless you give them the current database alias (available from schema_editor.connection.alias, where schema_editor is the second argument to your function).*

But the docs section that describes data migrations, it doesn't mention the alias at all:

from django.db import migrations

def combine_names(apps, schema_editor):
    # We can't import the Person model directly as it may be a newer
    # version than this migration expects. We use the historical version.
    Person = apps.get_model('yourappname', 'Person')
    for person in Person.objects.all():
        person.name = '%s %s' % (person.first_name, person.last_name)
        person.save()

class Migration(migrations.Migration):

    dependencies = [
        ('yourappname', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(combine_names),
    ]

标签： djangodjango-migrations