Apache pig 从JSON数组读取/提取$date字段值

我有一个文件，其中每一行都是一个JSON对象（实际上，它是stackoverflow的转储）。我能够轻松地将其加载到ApachePig中，但在从LastActivityDate读取/提取“$date”时遇到了问题。您能帮助我如何从“LastActivityDate”中获取$date值吗

以下是一个例子：

{ 
"_id" : { "$oid" : "506492073401d91fa7fdffbe" }, 
"Body" : "....", 
"ViewCount" : 7351, 
"LastEditorDisplayName" : "Rich B", 
"Title" : ".....", 
"LastEditorUserId" : 140328, 
"LastActivityDate" : { "$date" : 1314819738077 }, 
"LastEditDate" : { "$date" : 1313882544213 }, 
"AnswerCount" : 12, "CommentCount" : 19, 
"AcceptedAnswerId" : 7, 
"Score" : 83, 
"PostTypeId" : "question", 
"OwnerUserId" : 8, 
"Tags" : [ "c#", "winforms" ], 
"CreationDate" : { "$date" : 1217540572667 }, 
"FavoriteCount" : 13, "Id" : 4, 
"ForumName" : "stackoverflow.com" 
}

我得到以下错误：

grunt> entitleGen1 = FOREACH entitleGen GENERATE id, ViewCount, LastActivityDate#'$date' as "LastActivityDate" :chararray;
2015-12-01 05:42:19,840 [main] ERROR org.apache.pig.impl.PigContext - Undefined parameter : date