Mongo push objects - arrays

Question

I want to push an object to specify name of fields rather than array. I tried $push but I lose informations about field's name inserted in the array.
My collection is :
/* 1 */
{
"_id" : ObjectId("57614a7bd75df17df3013903"),
"O":"aa",
"D":"bb",
"month":1,
"year":2015,
"freq":5
}
/* 2 */
{
"_id" : ObjectId("57614a7bd75df17df3013904"),
"O":"aa",
"D":"bb",
"month":2,
"year":2015,
"freq":5
}
/* 3 */
{
"_id" : ObjectId("57614a7bd75df17df3013905"),
"O":"aa",
"D":"bb",
"month":1,
"year":2016,
"freq":5
}
I want to store all freq corresponding to fields : O and D.
Here is my expected output :
"_id" : ...,
"O" : "aa",
"D" : "bb",
"freq" : {
"2015" : {
"1" : 5,
"2":5
},
"2016" : {
"1" : 5
}
}
}
I tried this :
db.collection.aggregate([
{
'$group':
{
_id:{"O":"$O","D":"$D","Y":"$year"},
"freq" :{$push: "$freq"}
}
},
{
'$group':
{
_id:{"O":"$O","D":"$D"},
"freq" :{$push: "$freq"}
}
})]
but I got an array without informations of year or month.
Thank you

Answer 1

You have used two $group in your query
Your First group query is enough to build the data which you are expecting.
If we are executing the first query
db.stackoverflow.aggregate([
{
'$group':
{
_id:{"O":"$O","D":"$D","Y":"$year"},
"freq" :{$push: "$freq"}
}
}]);
then the result is
{ "_id" : { "O" : "aa", "D" : "bb", "Y" : 2016 }, "freq" : [ 5 ] }
{ "_id" : { "O" : "aa", "D" : "bb", "Y" : 2015 }, "freq" : [ 5, 5 ] }
Now if you execute your second $group query
db.stackoverflow.aggregate([
{
'$group':
{
_id:{"O":"$O","D":"$D"},
"freq" :{$push: "$freq"}
}
}])
then the result is
{ "_id" : { "O" : "aa", "D" : "bb" }, "freq" : [ 5, 5, 5 ] }
Reason:
The values fetched in the first $group query is not passed to the second $group query.
Solution:
Use $project available in the aggregation pipeline which passes along the documents with only the specified fields to the next stage in the aggregation pipeline. The specified fields can be existing fields from the input documents or newly computed fields.
https://docs.mongodb.com/manual/reference/operator/aggregation/project/
Here is the query to get your expected result
db.collection.aggregate([
{
'$group': {
_id: {
"o": "$o",
"d": "$d",
"year": "$year"
},
myArr: {
$push: {
year: "$year",
month: "$month",
freq: "$freq"
}
}
}
},
{
'$group': {
_id: {
"o": "$o",
"d": "$d"
},
myArr1: {
$push: {
year: "$year",
freq: "$myArr"
}
}
}
},
],
{
allowDiskUse: true
})