Mongodb group by and sum and get media with Map - mongodb

Question

I have these collections in my database:
Items:
{ "IdUser" : "1", "IdItem" : "1" },
{ "IdUser" : "1", "IdItem" : "2" },
{ "IdUser" : "1", "IdItem" : "3" },
{ "IdUser" : "2", "IdItem" : "4" },
{ "IdUser" : "2", "IdItem" : "5" },
{ "IdUser" : "4", "IdItem" : "6" },
{ "IdUser" : "5", "IdItem" : "7" }
Users
{ "_id" : "1", "DateRegister" : ISODate("2016-03-29T22:00:38.764+0000") },
{ "_id" : "2", "DateRegister" : ISODate("2014-03-29T22:00:38.764+0000") },
{ "_id" : "2", "DateRegister" : ISODate("2015-02-29T22:00:38.764+0000") },
{ "_id" : "4", "DateRegister" : ISODate("2013-01-29T22:00:38.764+0000") },
{ "_id" : "5", "DateRegister" : ISODate("2016-04-29T22:00:38.764+0000") }
How can I obtain this result but FILTERED with users registered after 2015:
Users with one item: 2
Users with two items: 1
Users with three items: 1
I have tried with that, but I don't know how to filter... Thanks!
db.collection.aggregate([
{
"$group": {
"_id": "$IdUser",
"count": {
"$sum": { "$cond": [{ "$gt": [ "$IdItem", null ] }, 1, 0 ] }
}
}
},
{
"$group": {
"_id": "$count",
"users": { "$push": "$_id" }
}
},
{
"$project": {
"_id": 0,
"number_of_items": "$_id",
"number_of_users": { "$size": "$users" }
}
}
])

Answer 1

You may want to utilize the $lookup operator to perform a join of the items collection with the users collection and then do a $match filter on the DateRegistered field before piping the main grouping operations.
Following this example + the links herein to the documentation will give you an idea:
db.items.aggregate([
{
"$lookup": {
"from": "users",
"localField": "IdUser",
"foreignField": "_id",
"as": "user"
}
},
{ "$match": { "user.DateRegister": { "$gt": new Date(2015, 11, 31) } } },
{
"$group": {
"_id": "$IdUser",
"count": {
"$sum": { "$cond": [{ "$gt": [ "$IdItem", null ] }, 1, 0 ] }
}
}
},
{
"$group": {
"_id": "$count",
"users": { "$push": "$_id" }
}
},
{
"$project": {
"_id": 0,
"number_of_items": "$_id",
"number_of_users": { "$size": "$users" }
}
}
])
In the event that your MongoDB server does not support the $lookup operator, you will then need a workaround where you split the operations on the different collections i.e.
get a list of user id's that match the given date range criteria, this could be done with the distinct() method on the users collection with the date query option.
use that list in the items collection aggregation pipeline within the $match operator initial step.
The following demonstrates this:
// use distinct to get the user id's list
var userIds = db.users.distinct("_id", { "DateRegister": { "$gt": new Date(2015, 11, 31) } })
// perform your aggregation with a filtered collection using the list from the above operations
db.items.aggregate([
{ "$match": { "IdUser": { "$in": userIds } } },
{
"$group": {
"_id": "$IdUser",
"count": {
"$sum": { "$cond": [{ "$gt": [ "$IdItem", null ] }, 1, 0 ] }
}
}
},
{
"$group": {
"_id": "$count",
"users": { "$push": "$_id" }
}
},
{
"$project": {
"_id": 0,
"number_of_items": "$_id",
"number_of_users": { "$size": "$users" }
}
}
])