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

Building on Mark's excellent answer - here's how to route both model read/write operations and migrations to different databases based on an attribute of the model

# app/models.py
class SomeModel(models.Model):
    class params:
        db = 'default'

class SomeOtherDbModel(models.Model):
    class params:
        db = 'otherdb'
    ...


# app/dbrouters.py
import app.models
allmodels = dict([(name.lower(), cls) for name, cls in app.models.__dict__.items() if isinstance(cls, type)])
...
class MyDBRouter(object):

    def db_for_read(self, model, **hints):
        """ reading model based on params """
        return getattr(model.params, 'db')

    def db_for_write(self, model, **hints):
        """ writing model based on params """
        return getattr(model.params, 'db')

    def allow_migrate(self, db, app_label, model_name = None, **hints):
        """ migrate to appropriate database per model """
        model = allmodels.get(model_name)
        return(model.params.db == db)


# app/settings.py
DATABASE_ROUTERS = ('app.dbrouters.MyDBRouter',)
...
DATABASES = {
    ...
    'otherdb': {
        ....
    }
}

See docs here: https://docs.djangoproject.com/en/3.0/topics/db/multi-db/#database-routers

The model_name argument is passed at runtime as the lower case of the model.__name__, hence why we built the lookup dictionary by casting this attribute to lower.

Migrations should then be run as

python3 manage.py migrate app --database default
python3 manage.py migrate app --database otherdb