def run_migrations_online():
    """Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    """

    # this callback is used to prevent an auto-migration from being generated
    # when there are no changes to the schema
    # reference: http://alembic.zzzcomputing.com/en/latest/cookbook.html
    def process_revision_directives(context, revision, directives):
        if getattr(config.cmd_opts, 'autogenerate', False):
            script = directives[0]
            if len(script.upgrade_ops_list) >= len(bind_names) + 1:
                empty = True
                for upgrade_ops in script.upgrade_ops_list:
                    if not upgrade_ops.is_empty():
                        empty = False
                if empty:
                    directives[:] = []
                    logger.info('No changes in schema detected.')

    conf_args = current_app.extensions['migrate'].configure_args
    if conf_args.get("process_revision_directives") is None:
        conf_args["process_revision_directives"] = process_revision_directives

    # for the direct-to-DB use case, start a transaction on all
    # engines, then run all migrations, then commit all transactions.
    engines = {
        '': {'engine': get_engine()}
    }
    for name in bind_names:
        engines[name] = rec = {}
        rec['engine'] = get_engine(bind_key=name)

    for name, rec in engines.items():
        engine = rec['engine']
        rec['connection'] = conn = engine.connect()

        if USE_TWOPHASE:
            rec['transaction'] = conn.begin_twophase()
        else:
            rec['transaction'] = conn.begin()

    try:
        for name, rec in engines.items():
            logger.info("Migrating database %s" % (name or '<default>'))
            context.configure(
                connection=rec['connection'],
                upgrade_token="%s_upgrades" % name,
                downgrade_token="%s_downgrades" % name,
                target_metadata=get_metadata(name),
                **conf_args
            )
            context.run_migrations(engine_name=name)

        if USE_TWOPHASE:
            for rec in engines.values():
                rec['transaction'].prepare()

        for rec in engines.values():
            rec['transaction'].commit()
    except:  # noqa: E722
        for rec in engines.values():
            rec['transaction'].rollback()
        raise
    finally:
        for rec in engines.values():
            rec['connection'].close()
Run migrations in 'online' mode.

    In this scenario we need to create an Engine
    and associate a connection with the context.

    
src/flask_migrate/templates/flask-multidb/env.py