MongoDB Filter only IF ANY - mongodb

Question

Mongo Playgound
Lets say I have these results:
A)
[
{_id: 1, Name: 'A', Price: 10, xx:0},
{_id: 2, Name: 'B', Price: 15, xx:0},
{_id: 3, Name: 'A', Price: 100, xx:1},
{_id: 4, Name: 'B', Price: 150, xx:1},
]
B)
[
{_id: 1, Name: 'A', Price: 10, xx:0},
{_id: 2, Name: 'B', Price: 15, xx:0},
]
I want to:
If exists at least one x:1, return all x:1 only
If there is none x:1, return all x:0
Should I do a MAP & FILTER on root docs? or some kind of MATCH with conditionals? or Redact?
Results desired Ex.:
A) Removed x:0 because exists x:1, so returned only x:1
[
{_id: 3, Name: 'A', xx:1},
{_id: 4, Name: 'B', xx:1},
]
B) Returned only x:0 as there are only x:0
[
{_id: 1, Name: 'A', xx:0},
{_id: 2, Name: 'B', xx:0},
]

Answer 1

Group the documents by the xx field and add the grouped docs to the docs array using $push.
Sort the docs by the _id field in descending order.
Limit the result to 1.
If there are documents with both xx: 0 and xx: 1 values, only the xx: 1 group would be returned since we're sorting in descending order and limiting the result to the first group. If there are no documents with xx: 1 but documents with xx: 0 exist, the first group would be xx: 0 which gets returned.
You can then use $unwind to return a document for each grouped document and $replaceRoot to lift the document to the root level.
db.collection.aggregate([
{
$group: {
_id: "$xx",
docs: {
$push: "$$ROOT",
}
}
},
{
$sort: {
_id: -1,
}
},
{
$limit: 1,
},
{
$unwind: "$docs"
},
{
$replaceRoot: {
newRoot: "$docs"
},
}
])
MongoPlayground
If there might be docs with an xx value other than 0 and 1, you should filter those out using $match before grouping the docs using $group.
db.collection.aggregate([
{
$match: {
xx: {
$in: [
0,
1
]
}
}
},
{
$group: {
_id: "$xx",
docs: {
$push: "$$ROOT",
}
}
},
{
$sort: {
_id: -1,
}
},
{
$limit: 1,
},
{
$unwind: "$docs"
},
{
$replaceRoot: {
newRoot: "$docs"
},
}
])
MongoPlayground