How can I group by ID and Month? - mongodb

Question

How can I group by ID and Month in MongoDB?
My data looks like this:
{
"_id" : ObjectId("597225c62e7cbfc9a0b099f8"),
"LogId" : NumberInt(17351963),
"EntryId" : NumberInt(22),
"Date" : "2013-08-11 00:00:00",
"LogTypeId" : NumberInt(6),
"Count" : NumberInt(1),
"EntryType" : NumberInt(1)
}
{
"_id" : ObjectId("597225c62e7cbfc9a0b099f9"),
"LogId" : NumberInt(17352356),
"EntryId" : NumberInt(23),
"Date" : "2013-08-11 00:00:00",
"LogTypeId" : NumberInt(6),
"Count" : NumberInt(2),
"EntryType" : NumberInt(1)
}
{
"_id" : ObjectId("597225c62e7cbfc9a0b099fa"),
"LogId" : NumberInt(17360483),
"EntryId" : NumberInt(28),
"Date" : "2013-08-11 00:00:00",
"LogTypeId" : NumberInt(6),
"Count" : NumberInt(1),
"EntryType" : NumberInt(1)
}
My simplified aggregation query runs without errors, but it doesn't group:
db.log.aggregate([
{"$group":{"_id":"$EntryId", "Count":{"$sum":"$Count"}}},
{"$sort": {"EntryId": 1}}
])
Ultimately, I want to group by EntryID and the month of the date column.

Answer 1

First of all "convert your strings to date" with a very simple operation:
let ops = [];
db.log.find().forEach(doc => {
ops.push({ "updateOne": {
"filter": { "_id": doc._id },
"update": { "$set": { "Date": new Date(doc.Date.replace(" ","T")) } }
}});
if ( ops.length >= 500 ) {
db.log.bulkWrite(ops);
ops = [];
}
});
if ( ops.length > 0 ) {
db.log.bulkWrite(ops);
ops = [];
};
Then run the new aggregate:
db.log.aggregate([
{ "$group": {
"_id": {
"EntryId": "$EntryId",
"year": { "$year": "$Date" },
"month": { "$month": "$Date" }
},
"Count": { "$sum": 1 }
}},
{ "$sort": { "_id": 1 } }
])
Also noting that even a "compound _id" like this one will sort correctly to it't numeric values.