Record Aggregation in Mongo across an Array - mongodb

Question

I have an array stored in each document/record in a mongo database and I need to compute a score for each element in this array and aggregate the scores by another field in the array element.
It's hard for me to explain what I am trying to do in english so here is a python example of what I am looking to do.
records = [
{"state": "a", "initvalue": 1, "data": [{"time": 1, "value": 2}, {"time": 2, "value": 4}]},
{"state": "a", "initvalue": 5, "data": [{"time": 1, "value": 7}, {"time": 2, "value": 9}]},
{"state": "b", "initvalue": 4, "data": [{"time": 1, "value": 2}, {"time": 2, "value": 1}]},
{"state": "b", "initvalue": 5, "data": [{"time": 1, "value": 3}, {"time": 2, "value": 2}]}
]
def sign(record):
return 1 if record["state"] == "a" else -1
def score(record):
return [{"time": element["time"], "score": sign(record) * (element["value"] - record["initvalue"])} for element in record["data"]]
scores = []
for record in records:
scores += score(record)
sums = {}
for score in scores:
if score["time"] not in sums:
sums[score["time"]] = 0
sums[score["time"]] += score["score"]
print '{:>4} {:>5}'.format('time', 'score')
for time, value in sums.iteritems():
print '{:>4} {:>5}'.format(time, value)
This computes a slightly different score function for state a and for state b and then aggregates the scores across each time entry.
Here is the result
time score
1 7
2 13
I am trying to figure out how to do this in mongo, without pulling the records into python and reinventing aggregation.
Thanks for the help!

Answer 1

Ok. I figured this out. Once I really understood how pipeline's work and about the condition function everything came together.
from pymongo import MongoClient
client = MongoClient()
result = client.mydb.foo.aggregate([
{'$project': {'_id': 0, 'data': 1, 'initvalue': 1, 'state': 1}},
{'$unwind': '$data'},
{'$project': {
'time': '$data.time',
'score': {'$multiply': [
{'$cond': [{'$eq': ['$state', 'a']}, 1, -1]},
{'$subtract': ['$data.value', '$initvalue']}
]}
}},
{'$group': {
'_id': '$time',
'score': {'$sum': '$score'}
}},
{'$project': {'_id': 0, 'time': '$_id', 'score': 1}}
])
for record in result['result']:
print record
This yields the desired result
{u'score': 13, u'time': 2}
{u'score': 7, u'time': 1}