Django - how to specify a database for a model?

Question

Is there a way to specify that a model (or app, even) should only ever use one particular database?

I am working with a legacy database that I don\'t want to change.

Answer 1

Tested with Django 2.2 and pytest.
Just to streamline a bit the excellent @chris-schon answer:

class LegacyDbModel(models.Model):

    class Meta:
        abstract = True
        _db = 'legacy_db_alias'


class LegacyDbRouter(object):

    def db_for_read(self, model, **hints):
        """ reading model based on params """
        if not hasattr(model, 'Meta'):
            return None
        return getattr(model.Meta, '_db', None)

    def db_for_write(self, model, **hints):
        """ writing model based on params """
        if not hasattr(model, 'Meta'):
            return None
        return getattr(model.Meta, '_db', None)

It has couple of minor benefits:

1. We don't need a params nested class for models from default DB.
2. We leverage Meta nested class which is already part of Django.
3. Router works without allow_migrate even for tests.

The only drawback is that we need to inherit from Meta explicitly:

class LegacyModel(LegacyDbModel):
    # ... docs, fields, etc.

    class Meta(LegacyDbModel.Meta):
        managed = False
        db_table = 'legacy_table'

But even that is more aligned with how Django works.
Remember:

Explicit is better than implicit.