How to update a field in nested array(two level at least) [duplicate]

the document is(I have simplified the structure):

    {
        "_id" : ObjectId("561f7896d3e4d2d8a6406baa"),
        "array" : [ 
            {
                "array" : [ 
                    {
                        "taskId" : 1,
                        "finished" : false
                    }
                ]
            }
        ]
    }  

I want to update the finished to true.
The query is

db.getCollection('JustTest').find({"array.array.taskId":1})

I tried db.getCollection('JustTest').update({"array.array.taskId":1},{$set:{"array.$.finished":true}}).But the result is

    {
        "_id" : ObjectId("561f7896d3e4d2d8a6406baa"),
        "array" : [ 
            {
                "array" : [ 
                    {
                        "taskId" : 1,
                        "finished" : false
                    }
                ],
                "finished" : true
            }
        ]
    }

And "array.$.array.$.finished":true is illegal and returns error msg.

I have read http://docs.mongodb.org/manual/reference/operator/update/positional/ .I also found a similar question How to update data in nested array in MongoDB but the answer is maybe naive.

I have no idea to update field in the nested array.Fix the position is not a good idea because the arrays(inner or outer) may be pushed or popped(from middle).making two collection can work but query will be difficult.