MongoDB Aggregation: grouping data within date groups - mongodb

Question

I have many documents in a MongoDB database which look like the following four documents (note the first 3 are Feb 2017 and the last one is March 2017):
{"_id": 0,
"date": ISODate("2017-02-01T00:00:00Z),
"item": "Basketball",
"category": "Sports"}
{"_id": 1,
"date": ISODate("2017-02-13T00:00:00Z),
"item": "Football",
"category": "Sports"}
{"_id": 2,
"date": ISODate("2017-02-22T00:00:00Z),
"item": "Massage",
"category": "Leisure"}
{"_id": 3,
"date": ISODate("2017-03-05T00:00:00Z),
"item": "Golf club",
"category": "Sports"}
I'm trying to group the items by MONTH/YEAR and within that, group the items by CATEGORY. So the aggregation pipeline should return something that looks like this for the four docs above:
{"_id": {
"month": 2,
"year": 2017
},
"data": [
{"category": "Sports",
"items": ["Basketball", "Football"]
},
{"category": "Leisure",
"items": ["Massage"]
}
]
},
{"_id": {
"month": 3,
"year": 2017
},
"data": [
{"category": "Sports",
"items": ["Golf Club"]
}
]
}
I also want the returned cursor to be in order with year as the primary sort and month as the secondary sort.

Answer 1

Figured it out. Here's the answer using the pymongo api:
from bson.son import SON
cursor = db.collection.aggregate([
{'$group': {
'_id': {'month': {'$month': '$date'},
'year': {'$year': '$date'},
'$category': '$category'},
'items': {'$push': '$item'}
}},
{'$group': {
'_id': {'month': '_id.month',
'year': '_id.year'}
'data': {
'$push': {
'category': '$_id.category',
'items': '$items'
}
}
}},
{'$sort': SON([('_id.year', 1), ('_id.month', 1)])}
])
my_data = list(cursor)