coi/app/coi/routers.py

class DatabaseRouter:
    """
    A router to control all database operations for specific apps
    targeting separate databases.
    """

    # Define app labels that use the secondary database
    route_app_labels = {"legacy"}

    def db_for_read(self, model, **hints):
        """
        Direct read operations for legacy_app models to OB2011DB.
        """
        if model._meta.app_label in self.route_app_labels:
            return "OB2011DB"
        return "default"

    def db_for_write(self, model, **hints):
        """
        Direct write operations for legacy_app models to OB2011DB.
        """
        if model._meta.app_label in self.route_app_labels:
            return "OB2011DB"
        return "default"

    def allow_relation(self, obj1, obj2, **hints):
        """
        Allow relations if both objects belong to the same database.
        """
        db_set = {"default", "OB2011DB"}
        if obj1._state.db in db_set and obj2._state.db in db_set:
            return True
        return None

    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Ensure migrations are applied only to the appropriate database.
        """
        if app_label in self.route_app_labels:
            return db == "OB2011DB"
        return db == "default"