嵌套文档MongoDB查询的PHP语法

Question

文档样本：

Activity wfApp = new MyProject.Workflow.EventWorkflow();

PHP代码：

stdClass Object
(
    [_id] => MongoDB\BSON\ObjectId Object
        (
            [oid] => xxxxxxxxxxx
        )

    [sent_date_time_stamp] => MongoDB\BSON\UTCDateTime Object
        (
            [milliseconds] => xxxxxxxxxxxxxxx
        )


    [sent_data] => {"switchId":"db_sw_001","isQuery":true,"command":"6541"}

    [device_identifier] => stdClass Object
        (
            [$ref] => device
            [$id] => db_sw_001
        )

)

上述查询返回NULL

请帮助