How to group by element in object of nested array - mongodb

Question

I have nested arrays and objects. I want to filter out each medicine separately. Such as document:
{
person: "X",
treatment: [
{
id: 1,
medication: [
{
name: "A"
},
{
name: "B"
}
]
},
{
id: 2,
medication: [
{
name: "A"
},
{
name: "C"
}
]
}
]
},
{
person: "Y",
treatment: [
{
id: 1,
medication: [
{
name: "B"
},
{
name: "C"
}
]
}
]
}
How could I get a distinct list of top medication? Like this: A:2, B:2, C:2

Answer 1

You need to use $unwind which outputs one document for each element in the array. As there are nested array's you'll need to apply $unwind twice.
E.g. for a collection named medicine:
db.medicine.aggregate([
{
$unwind : "$treatment"
},
{
$unwind : "$treatment.medication"
},
{
$group : { _id : "$treatment.medication.name", total: { $sum: 1 } }
},
{
$replaceRoot: {
newRoot: {
$arrayToObject: {
$let: {
vars: { data: [ { k: "$_id", v: "$total" } ] },
in: "$$data"
}
}
}
}
}
])
Outputs:
{ "C" : 2 }
{ "B" : 2 }
{ "A" : 2 }
Note that when unwinding nested arrays, many documents can be generated if there are large arrays, so you might want to check this question Consequences of using $unwind on nested arrays?