(mongodb >=1.17.0)
MongoDB\BSON\Document::offsetGet — Returns the value of a key in the document
keyThe key to retrieve from the document.
Returns the value associated with the given key. If the key is not present in the document, an exception is thrown.
注意: When encountering a value encoded as 64-bit integer in the BSON document, the return value of this method will be a MongoDB\BSON\Int64 instance.