MongoDB - double $group make 2nd ObjectID a nested document - mongodb

Question

I have a collection that after unwind has this structure (I've deleted information which I think is not relevant to the question)
{
"_id" : ObjectId("1"),
"Members" : {
"City" : "New York"
},
"Group_name" : "Group A"
}
{
"_id" : ObjectId("2"),
"Members" : {
"City" : "Seattle"
},
"Group_name" : "Group A"
}
{
"_id" : ObjectId("3"),
"Members" : {
"City" : "Seattle"
},
"Group_name" : "Group A"
}
{
"_id" : ObjectId("4"),
"Members" : {
"City" : "New York"
},
"Group_name" : "Group B"
}
{
"_id" : ObjectId("5"),
"Members" : {
"City" : "Los Angeles"
},
"Group_name" : "Group B"
}
{
"_id" : ObjectId("6"),
"Members" : {
"City" : "Los Angeles"
},
"Group_name" : "Group B"
}
I have used double Object Id to get a result like this:
{ "_id" : { "group" : "A", "city" : "New York" }, "totalMembers" : 1 }
{ "_id" : { "group" : "A", "city" : "Seattle" }, "totalMembers" : 2 }
{ "_id" : { "group" : "B", "city" : "New York" }, "totalMembers" : 1 }
{ "_id" : { "group" : "B", "city" : "Los Angeles" }, "totalMembers" : 2 }
I want to be able to obtain a document with the following structure:
{
"_id" : "A",
"Cities" : {
"New York" : 1,
"Seattle" : 2
}
}
{
"_id" : "B",
"Cities" : {
"New York" : 1,
"Los Angeles" : 2
}
}
This is my code so far, I haven't been able to group by 'group' and then by 'city'
db.Users_Group.aggregate([
{"$unwind":"$Members"},
{"$group":{"_id":{"group":"$Group_Name","City":"$Members.City"},"totalUsers":{"$sum":1}}},
{"$group":{"_id":"$_id.grupo","total":{"$sum":1}}}
] )
With this I get the sum of all members in that group not separated by cities. How can I nest a document of the cities with the total of users of that city within each group? Appreciate any help on this. Thanks in advance.

Answer 1

You need to run one more $group and prepare the data for $arrayToObject which takes an array of k-v pairs:
db.collection.aggregate([
// your current aggregation stages
{
$group: {
_id: "$_id.group",
Cities: { $push: { k: "$_id.city", v: "$totalMembers" } }
}
},
{
$project: {
_id: 1,
Cities: { $arrayToObject: "$Cities" }
}
}
])
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