Mongodb - group by same value in different fields in different documents - mongodb

Question

I have documents with common values in different fields that I want to group by that value. Simplified records are:
{ _id:1,
"Home" : "A",
"Away" : "B" }
{ _id:2,
"Home" : "B",
"Away" : "C" }
{ _id:3,
"Home" : "C",
"Away" : "A" }
{ _id:4,
"Home" : "C",
"Away" : "B" }
{ _id:5,
"Home" : "A",
"Away" : "C" }
I am trying to get an aggregate group result that includes, for example, the value "A" whether it appears in a document in the field "Home", or the field "Away". The result I want is:
{"_id": "A", "count": 3},
{"_id": "B", "count": 3},
{"_id": "C", "count": 4}
Grouping by either "Home" or "Away" is no problem but that wouldn't give me all the records, as shown below, I wouldn't get a count of records where "A" or "B" or "C" was in the "Home" field:
{$group:
{_id: "$Away"} etc... }
I have tried using $cond from other posts here as follows:
$group : {
_id : {
$cond : [{
$gt : [ "$Away", null]
}, "$Home"]
}
}
Also tried an $or which is pretty obviously wrong since it will only find the same value for Away and Home fields within each document (which is never the case):
$group : {
_id : {
$or : [ "$Away", "$Home"]
}
}
I'm stuck and not sure if this is even possible; to group on a value that may be in different fields in different documents.

Answer 1

You can create an object to use $objectToArray and $unwind and then group like this:
Create object using $set and the same values ($Home and $Away)
Use project to not pass these values to the next stage. There are no neccesary, you have the object.
Then $objectToArray to do $unwind and get every value
And last $group by property v generated by $objectToArray.
db.collection.aggregate([
{
"$set": {
"obj": {
"Home": "$Home",
"Away": "$Away"
}
}
},
{
"$project": {"Away": 0,"Home": 0}
},
{
"$set": {"obj": {"$objectToArray": "$obj"}}
},
{
"$unwind": "$obj"
},
{
"$group": {
"_id": "$obj.v",
"count": {"$sum": 1}
}
}
])
Example here