MongoDB nested group? - mongodb

Question

I'm trying to implement a nested group query in mongodb and I'm getting stuck trying to add the outer group by. Given the below (simplified) data document:
{
"timestamp" : ISODate(),
"category" : "movies",
"term" : "my movie"
}
I'm trying to achieve a list of all categories and within the categories there should be the top number of terms. I would like my output something like this:
[
{ category: "movies",
terms: [ { term: "movie 1", total: 5000 }, { term: "movie 2", total: 200 } ... ]
},
{ category: "sports",
terms: [ { term: "football 1", total: 4000 }, { term: "tennis 2", total: 250 } ... ]
},
]
My 'inner group' is as shown below, and will get the top 5 for all categories:
db.collection.aggregate([
{ $match : { "timestamp": { $gt: ISODate("2014-08-27") } } },
{ $group : { _id : "$term", total : { $sum : 1 } } },
{ $sort : { total : -1 } },
{ $limit: 5 }
]);
// Outputs:
{ "_id" : "movie 1", "total" : 943 }
{ "_id" : "movie 2", "total" : 752 }
How would I go about implementing the 'outer group'?
Additionally sometimes the above aggregate]ion returns a null value (not all documents have a term value). How do I go about ignoring the null values?
thanks in advance

Answer 1

You will need two groups in this case. The first group generates a stream of documents with one document per term and category:
{ $group : {
_id : {
category: "$category",
term: "$term",
},
total: { $sum : 1 }
}
}
A second group will then merge all documents with the same term into one, using the $push operator to merge the categories into an array:
{ $group : {
_id : "$_id.category",
terms: {
$push: {
term:"$_id.term",
total:"$total"
}
}
}
}

Answer 2

Query:
db.getCollection('orders').aggregate([
{$match:{
tipo: {$regex:"[A-Z]+"}
}
},
{$group:
{
_id:{
codigo:"1",
tipo:"$tipo",
},
total:{$sum:1}
}
},
{$group:
{
_id:"$_id.codigo",
tipos:
{
$push:
{
tipo:"$_id.tipo",
total:"$total"
}
},
totalGeneral:{$sum:"$total"}
}
}
]);
Response:
{
"_id" : "1",
"tipos" : [
{
"tipo" : "TIPO_01",
"total" : 13.0
},
{
"tipo" : "TIPO_02",
"total" : 2479.0
},
{
"tipo" : "TIPO_03",
"total" : 12445.0
},
{
"tipo" : "TIPO_04",
"total" : 12445.0
},
{
"tipo" : "TIPO_05",
"total" : 21.0
},
{
"tipo" : "TIPO_06",
"total" : 21590.0
},
{
"tipo" : "TIPO_07",
"total" : 1065.0
},
{
"tipo" : "TIPO_08",
"total" : 562.0
}
],
"totalGeneral" : 50620.0
}