MongoDB Update Array element

MongoDB Update Array element

I have a document structure like
{
"_id" : ObjectId("52263922f5ebf05115bf550e"),
"Fields" : [
{
"Field" : "Lot No",
"Rules" : [ ]
},
{
"Field" : "RMA No",
"Rules" : [ ]
}
]
}
I have tried to update by using the following code to push into the Rules
Array which will hold objects.
db.test.update({
"Fields.Field":{$in:["Lot No"]}
}, {
$addToSet: {
"Fields.Field.$.Rules": {
'item_name': "my_item_two",
'price':1
}
}
}, false, true);
But I get the following error can't append to array using string field
name [Field]
How do I do the update? I have searched thru similiar posts but nothing is
clicking. Help.