Order by Array of documents in aggregation mongodb - mongodb

Question

I have a collection with documents like these
{
"Day" : 1
"Name" : "Fred",
"Badges" : ["STAR", "FAST"]
}
{
"Day" : 2
"Name" : "Fred",
"Badges" : ["STAR", "NEWBIE", "SLOW"]
}
{
"Day" : 3
"Name" : "Fred",
"Badges" : ["STAR", "NEWBIE", "FAST"]
}
{
"Day" : 1
"Name" : "Frank",
"Badges" : ["STAR","GO"]
}
{
"Day" : 2
"Name" : "Frank",
"Badges" : ["STAR"]
}
I aggregated these documents for all month in order to return a statistic like this
{
Name : "Fred"
Badges : [{Name : "STAR", Count : 3},
{Name : "NEWBIE", Count : 2},
{Name : "FAST", Count : 2},
{Name : "SLOW", Count : 1}
]
}
and the same thing for Frank.
{
Name : "Fred"
Badges : [{Name : "STAR", Count : 2},
{Name : "GO", Count : 1}
]
}
I have a problem with the sort .
How can I order for badge value STAR ?
Could I change the aggregation query in order to produce different result ?
I think that a structure like this can be good solution but I don't understand how to obtain it
{Name : "Fred"
STAR : 2
GO : 1}
Thanks

Answer 1

db.a.aggregate([
{$unwind: "$Badges"},
{$group: {_id: {name: "$Name", badge: "$Badges"}, numberOfBadges: { $sum: 1}}},
{$project: {_id: 0,name: "$_id.name", badge: "$_id.badge", numberOfBadges:1, orderField: {$cond: [{$eq: ["$_id.badge", "STAR"]}, "$numberOfBadges", 0]}}},
{$sort: {orderField:-1}}])
As far as I know you cannot have dynamic field names in aggregation framework so you can't have a field name as STAR. But you can make a trick by adding a new field to your document such as orderField and set the number of numberOfBadges to that document if the badge is STAR, otherwise set 0 to that field. Then sort on orderField.