gistfile1.txt

> db.post.insert({comment: []})
WriteResult({ "nInserted" : 1 })
> db.post.update({}, {$push: {'comment': {text: '1', child:[]}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.post.update({}, {$push: {'comment': {text: '2', child:[]}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.post.update({}, {$push: {'comment.0.child': {text: '3', child:[]}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.post.update({}, {$push: {'comment.0.child.0.child': {text: '4', child:[]}}})
WriteResult({ "nMatched" : 1, "nUpserted" : 0, "nModified" : 1 })
> db.post.findOne()
{
    "_id" : ObjectId("55eac3ab9363046398e59c70"),
    "comment" : [
        {
            "text" : "1",
            "child" : [
                {
                    "text" : "3",
                    "child" : [
                        {
                            "text" : "4",
                            "child" : [ ]
                        }
                    ]
                }
            ]
        },
        {
            "text" : "2",
            "child" : [ ]
        }
    ]
}