MongoDB two groups Aggregate - mongodb

Question

Aggregation operations process data records and return computed results. Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result. MongoDB provides three ways to perform aggregation: the aggregation pipeline, the map-reduce function, and single purpose aggregation methods.
I would like to transform that :
{
"_id" : ObjectId("5836b919885383034437d4a7"),
"Identificador" : "G-3474",
"Miembros" : [
{
"_id" : ObjectId("5836b916885383034437d238"),
"Nombre" : "Pilar",
"Email" : "pcarrillocasa#gmail.es",
"Edad" : 24,
"País" : "España",
"Tipo" : "Usuario individual",
"Apellidos" : "Carrillo Casa",
"Teléfono" : 637567234,
"Ciudad" : "Santander",
"Identificador" : "U-3486",
"Información_creación" : {
"Fecha_creación" : {
"Mes" : 4,
"Día" : 22,
"Año" : 2016
},
"Hora_creación" : {
"Hora" : 15,
"Minutos" : 34,
"Segundos" : 20
}
}
}
}
into that
{
"Nombre_Grupo" : "Amigo invisible"
"Ciudades" : [
{
"Ciudad" : "Madrid",
"Miembros": 30
},
{
"Ciudad" : "Almería",
"Miembros": 10
}
{
"Ciudad" : "Badajoz",
"Miembros": 20
}
]
}
with MongoDB.
I tried with that:
db.Grupos_usuarios.aggregate([
{ $group: { _id: "$Nombre_Grupo",total: { $sum: "$amount" } },
$group: { _id: "$Ciudad",total: { $sum: "$amount" } } }
])
but I could not get what I needed.
May somebody help me to know what I am doing bad?

Answer 1

The following aggregation gets the output you are looking for.
The $unwind stage deconstructs an array field from the input documents to output a document for each element. These documents are used to group by the Miembros.Ciudad and get the total Miembros for each Ciudad. In the second group stage we Pivot data to get all the Ciudades from the previous grouping into an array. The last $project is for formatting the output.
db.test.aggregate( [
{
$unwind: "$Miembros"
},
{
$group: {
_id: "$Miembros.Ciudad",
total: { $sum: 1 }
}
},
{
$group: {
_id: "Amigo invisible",
Ciudades: { $push: { Ciudad: "$_id", Miembros: "$total"} }
}
},
{
$project: {
Nombre_Grupo: "$_id",
Ciudades: 1,
_id: 0
}
}
] )