mongo query select only first of month - mongodb

Question

is it possible to query only the first (or last or any single?) day of the month of a mongo date field.
i use the $date aggregation operators regularly but within a $group clause.
basically i have field that is already aggregated (averaged) for each day of the month. i want to select only one of these days (with the value as a representative of the entire month.)
following is a sample of a record set from jan 1, 2014 to feb 1, 2015 with price as the daily price and 28day_avg as the trailing monthly average for 28 days.
{ "date" : ISODate("2014-01-01T00:00:00Z"), "_id" : ObjectId("533b3697574e2fd08f431cff"), "price": 59.23, "28day_avg": 54.21}
{ "date" : ISODate("2014-01-02T00:00:00Z"), "_id" : ObjectId("533b3697574e2fd08f431cff"), "price": 58.75, "28day_avg": 54.15}
...
{ "date" : ISODate("2015-02-01T00:00:00Z"), "_id" : ObjectId("533b3697574e2fd08f431cff"), "price": 123.50, "28day_avg": 122.25}
method 1.
im currently running an aggregation using $month data (and summing the price) but one issue is im seeking to retrieve the underlying date value ISODate("2015-02-01T00:00:00Z") versus the 0,1,2 value that comes with several of the date aggregations (that loop at the first of the week, month, year). mod(28) on a date?
method 2
i'd like to simply pluck out a single record of the 28day_avg as representative of the period. the 1st of the month would be adequate
the desired output is...
_id: ISODate("2015-02-01T00:00:00Z"), value: 122.25,
_id: ISODate("2015-01-01T00:00:00Z"), value: 120.78,
_id: ISODate("2014-12-01T00:00:00Z"), value: 118.71,
...
_id: ISODate("2014-01-01T00:00:00Z"), value: 53.21,
of course, the value will vary from method 1 to method 2 but that is fine. one is 28 days trailing while the other will account for 28, 30, 31 day months...dont care about that so much.
A non-agg is ok but also doesnt work. aka {"date": { "$mod": [ 28, 0 ]} }

Answer 1

To pick the first of the month for each month (method 2), use the following aggregation:
db.test.aggregate([
{ "$project" : { "_id" : "$date", "day" : { "$dayOfMonth" : "$date" }, "28day_avg" : 1 } },
{ "$match" : { "day" : 1 } }
])
You can't use an index for the match, so this is not efficient. I'd suggest adding another field to each document that holds the $dayOfMonth value, so you can index it and do a simple find:
{
"date" : ISODate("2014-01-01T00:00:00Z"),
"price" : 59.23,
"28day_avg" : 54.21,
"dayOfMonth" : 1
}
db.test.ensureIndex({ "dayOfMonth" : 1 })
db.test.find({ "dayOfMonth" : 1 }, { "_id" : 0, "date" : 1, "28day_avg" : 1 })