Django database routers for multiple databases

                  import random


class PrimaryReplicaRouter:
    """Route reads to replicas, writes to primary."""

    def db_for_read(self, model, **hints):
        """Direct read queries to a random replica."""
        return random.choice(['replica1', 'replica2'])

    def db_for_write(self, model, **hints):
        """Direct write queries to primary database."""
        return 'default'

    def allow_relation(self, obj1, obj2, **hints):
        """Allow relations if both objects are in same db."""
        db_set = {'default', 'replica1', 'replica2'}
        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 only run on primary."""
        return db == 'default'