MongoDB Aggregation Data - mongodb

Question

I am new to Mongo Aggregation and have this data
{
"_id": {
"$oid": "5654a8f0d487dd1434571a6e"
},
"ValidationDate": {
"$date": "2015-11-24T13:06:19.363Z"
},
"DataRaw": " WL 00100100012015-08-28 02:44:17+0000+ 16.81 8.879 1084.00",
"ReadingsAreValid": true,
"locationID": " WL 001",
"Readings": {
"pH": {
"value": 8.879
},
"SensoreDate": {
"value": {
"$date": "2015-08-28T02:44:17.000Z"
}
},
"temperature": {
"value": 16.81
},
"Conductivity": {
"value": 1084
}
},
"HMAC":"ecb98d73fcb34ce2c5bbcc9c1265c8ca939f639d791a1de0f6275e2d0d71a801"
}
My goal is to calculate the average temperature for every two hours interval, then per month, year and week. I have tried various queries but no luck. This is what i have tried so far
data.aggregate([{"$unwind":"$Readings"},
{"$project":{"HourRecord":{"$hour":"Readings.SensoreDate.value"},
"YearRecord":{"$year":"$Readings.SensoreDate.value"}}},
{'$group' : {'_id' : "$locationID",
'AverageTemp' : { '$avg' : '$Readings.temperature.value'}}}
])
and i got an empty results like this
{u'ok': 1.0, u'waitedMS': 0L, u'result': [{u'AverageTemp': None, u'_id': None}]}
I have tried several other combinations but still got empty results.
The following Queries Return the required result per hour, month etc but how to i group by two hours interval instead of one hour
test_Agg.aggregate([{"$unwind":"$Readings"},
{"$project":{ "HourRecord": { "$hour":"$Readings.SensoreDate.value"},
"YearRecord": {"$year":"$Readings.SensoreDate.value"},
"MonthRecord": {"$month":"$Readings.SensoreDate.value"},
"locationID" : 1,
"Readings.pH.value":1,
'Readings.temperature.value' : 1}
},
{'$group' : {'_id' :"$HourRecord",
'AverageTemp' : { '$avg' : '$Readings.temperature.value'}}
}])

Answer 1

The second pipeline $project in the query didn't project the required values used in the subsequent pipelines. Hence, the result was empty.
The required fields have been added in the $project. Try the below query.
db.temperature.aggregate([{"$unwind":"$Readings"},
{"$project":{ "HourRecord": { "$hour":"$Readings.SensoreDate.value"},
"YearRecord": {"$year":"$Readings.SensoreDate.value"},
locationID : 1,
'Readings.temperature.value' : 1}
},
{'$group' : {'_id' : "$locationID",
'AverageTemp' : { '$avg' : '$Readings.temperature.value'}}}
]);
Output for the one document provided in the post:-
{
"_id" : " WL 001",
"AverageTemp" : 16.81
}