pipeline是不是有bug?

我在云函数中写lookup,编辑器是3.4.3-alpha

.lookup({  
                from: 'uni-id-users',  
                let: {  
                    order_user_id: '$user_id',  
                    order_platform_id: '$platform_id',  
                    order_map_arr: '$map_arr'  
                },  
                pipeline: $.pipeline()  
                    .match(dbCmd.expr($.and([  
                        $.eq(['$platform_userid', '$$order_user_id']),  
                        $.eq(['$platform_id', '$$order_platform_id']),  
                        $.eq(['$map_id', '$$order_map_arr'])  
                    ])))  
                    .project({  
                        _id: 0,  
                        mappoints: 1  
                    })  
                    .done(),  
                as: 'map_point'  
            })

一直给我报
$ is not defined

.lookup({  
                from: 'uni-id-users',  
                let: {  
                    order_user_id: '$user_id',  
                    order_platform_id: '$platform_id',  
                    order_map_arr: '$map_arr'  
                },  
                pipeline:[],  
                as: 'map_point'  
            })

就没任何问题没有任何问题