MongoDB group data melt - mongodb

Question

Say I have a small dataset:
[
{"A": 0, "B": 0, "X": 100, "Y": 100},
{"A": 1, "B": 0, "X": 50, "Y": 55},
{"A": 0, "B": 1, "X": 25, "Y": 30},
{"A": 1, "B": 1, "X": 1, "Y": 6}
]
I also have a pipeline where the final stage is a group:
[
{
"$group": {
"_id": {
"classification1": {
"$eq": ["$A", 1]
},
"classification2": {
"$eq": ["$B", 1]
}
},
"countX": {"$sum": "$X"},
"countY": {"$sum": "$Y"}
}
}
]
The output of this pipeline:
[
{"_id": {"classification1": false, "classification2": false}, "countX": 100, "countY": 100},
{"_id": {"classification1": true, "classification2": false}, "countX": 50, "countY": 55},
{"_id": {"classification1": false, "classification2": true}, "countX": 25, "countY": 30},
{"_id": {"classification1": true, "classification2": true}, "countX": 1, "countY": 6}
]
What pipeline steps would I need to reach a melted format like this?
[
{"name": "classification1", "countX": 51, "countY": 61},
{"name": "classification2", "countX": 26, "countY": 36}
]
Note that this transformation counts document 1 from the previous stage zero times, and counts document 4 twice (since both conditions are false, or both are true).
I have written a Javascript function for this, but Javascript functions cannot be invoked from the pipeline (aggregation pipelines must be serializable). Unfortunately, that means I have to unload the data from the DB, run the script on the data, and then load the transformed data back in as a temporary collection to finish the rest of the pipeline after this stage.
Any assistance is greatly appreciated.

Answer 1

I did some reading on facets. Somewhat verbose, but this query provides melted data in the proper format:
[
{
"$group": {
"_id": {
"classification1": {
"$eq": ["$A", 1]
},
"classification2": {
"$eq": ["$B", 1]
}
},
"countX": {"$sum": "$X"},
"countY": {"$sum": "$Y"}
}
},
{
"$facet": {
"classification1": [
{"$match": {"_id.classification1": true}},
{"$group": {"_id": null, "X": {"$sum": "$countX"}, "Y": {"$sum": "$countY"}}},
{"$addFields": {"name": "classification1"}}
],
"classification2": [
{"$match": {"_id.classification2": true}},
{"$group": {"_id": null, "X": {"$sum": "$countX"}, "Y": {"$sum": "$countY"}}},
{"$addFields": {"name": "classification2"}}
]
}
},
{
"$project": {"combine": {"$setUnion": ["$classification1", "$classification2"]}}
},
{
"$unwind": "$combine"
},
{
"$replaceRoot": {"newRoot": "$combine"}
},
{
"$project": {"_id": 0}
}
]