Nested grouping with MongoDB - mongodb

Question

Given a database the form of
[
{ gender: "m", age: 1, name: "A" },
{ gender: "f", age: 2, name: "B" },
{ gender: "m", age: 3, name: "C" },
{ gender: "f", age: 1, name: "D" },
{ gender: "m", age: 2, name: "E" },
{ gender: "f", age: 3, name: "F" },
{ gender: "m", age: 1, name: "G" },
{ gender: "f", age: 2, name: "H" },
{ gender: "m", age: 3, name: "I" },
{ gender: "f", age: 1, name: "J" }
]
I want to first group by age and secondly group by gender so that I get a nested result looking something like
[{
_id: "1",
children: [
{ _id: "f" },
{ _id: "m" }
]
}, {
_id: "2",
children: [
{ _id: "f" },
{ _id: "m" }
]
}, {
_id: "3",
children: [
{ _id: "f" },
{ _id: "m" }
]
}]
Here is what I tried so far:
db.example.aggregate(
{ $group: { _id: "$age", children: { $addToSet: {
age: "$age", gender: "$gender", name: "$name"
}}}},
{ $group: { _id: "$children.gender"}}
)
But this returns an {_id: null} as its result. Is this possible and in case yes, how?

Answer 1

Something like this should do it;
db.example.aggregate(
{
$group: {
_id: { age: "$age", gender: "$gender" },
names: { $addToSet: "$name" }
}
},
{
$group: {
_id: { age: "$_id.age" },
children: { $addToSet: { gender: "$_id.gender", names:"$names" } }
}
}
)
...which gives the result;
{
"_id" : {
"age" : 1
},
"children" : [
{ "gender" : "m", "names" : [ "G", "A" ] },
{ "gender" : "f", "names" : [ "J", "D" ] }
]
},
...
If you want the age as _id as in your example, just replace the second grouping's _id by;
_id: "$_id.age",