You need to write a database router like this one:
class MyModelRouter:
def db_for_read(self, model, **hints):
if model._meta.app_label == 'myapp':
return 'second_db'
return None
def db_for_write(self, model, **hints):
if model._meta.app_label == 'myapp':
return 'second_db'
return None
def allow_relation(self, obj1, obj2, **hints):
if obj1._meta.app_label == 'myapp' or obj2._meta.app_label == 'myapp':
return True
return None
def allow_migrate(self, db, app_label, model_name=None, **hints):
if app_label == 'myapp':
return db == 'second_db'
return None