MongoDB group by subdocument counts and year - mongodb

Question

Here is a sample of my document from collection called products:
{
"_id" : "B000KIT6LQ",
"brand" : "unknown",
"category" : "Electronics",
"price" : "11.99",
"title" : "Scosche KA2067B 2005..."
"reviews" : [
{
"date" : ISODate("1969-12-31T23:59:59Z"),
"score" : 5,
"user_id" : "AK7M5Y7E9O3L7",
"sentiment" : 0.5,
"text" : "Bought this so I ...",
"user_gender" : "female",
"voted_total" : 0,
"voted_helpful" : 0,
"user_name" : "Alex",
"summary" : "It is what it is"
},
{
"date" : ISODate("1969-12-31T23:59:59Z"),
"score" : 5,
"user_id" : "A26VRLMPEA8IDR",
"sentiment" : 0.352,
"text" : "Years ago I worked as an...",
"user_gender" : "male",
"voted_total" : 0,
"voted_helpful" : 0,
"user_name" : "Jack R. Smith",
"summary" : "Great Kit"
},
{
"date" : ISODate("1969-12-31T23:59:59Z"),
"score" : 4,
"user_id" : "A1TGBDVX3QXCRH",
"sentiment" : 0.19318181818181818,
"text" : "This insert works great in my ...",
"user_gender" : "female",
"voted_total" : 0,
"voted_helpful" : 0,
"user_name" : "J. Reed",
"summary" : "Fits great in my 2006 Spectra5"
}
]
}
I have many documents with multiple categories. I am trying to create a mongo query which will result in all categories with the number of reviews (subdocument) per year. I have to group by categories and year, and get the count for number of reviews.
This is the query that I have got so far:
db.products.aggregate([
{ $unwind : "$reviews" },
{ $group: {
_id: {category: "$category", date: "$reviews.date.getFullYear()"},
count: { $sum: 1 }}},
{$sort:{"count": -1}}
])
For some reason the getFullYear() method is not working for me. If I group by $reviews.date I get the results.
Any pointers on getting the query right is appreciated.
Thanks

Answer 1

You can't use JavaScript functions like getFullYear() in your aggregate pipeline, you need to use the equivalent aggregation Date operator, which in this case is $year.
db.products.aggregate([
{ $unwind : "$reviews" },
{ $group: {
_id: {category: "$category", date: {$year: "$reviews.date"}},
count: { $sum: 1 }}},
{$sort:{"count": -1}}
])