Mongodb grouby by and sum and get media - mongodb

Question

I have this collection in my database:
{ "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" }
How can I obtain this result:
Users with one item: 2
Users with two items: 1
Users with three items: 1

Answer 1

You need to first $group your documents by IdUser then count the number of time each IdUser appear in your collection using the $sum accumulator operator. This allows you in the next stage to group your documents by "count" and return the count for "user" with same number of "items".
db.items.aggregate([
{ "$group": {
"_id": "$IdUser",
"count": { "$sum": 1 }
}},
{ "$group": {
"_id": "$count",
"nUsers": { "$sum": 1 }
}}
])

Answer 2

Using the aggregate() method will give you the desired result though the output documents would be different in that instead of having a key/value pair, you have two different fields with values that show the number of users and their respective
item count. The following aggregation pipeline explains this:
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" }
}
}
])
In the above, you $group all the documents by the user key to get their counts, taking into consideration documents that may not have the item field, which would be discounted in the aggregate. A further $group operation is necessary to then get the users per count, in the form of an array.
The last step in the pipeline $project serves to reshape the final output so that you get the following output (with the sample documents supplied with the question):
{ "number_of_items" : 1, "number_of_users" : 2 }
{ "number_of_items" : 3, "number_of_users" : 1 }
{ "number_of_items" : 2, "number_of_users" : 1 }