Say we have the following docs:
{a: 1, b: 2},
{a: 2, b: 0},
{a: 3, b: 1}
I want a query that will return:
[0, 1, 2, 3]
I want to know if there is a way to do this faster than:
a, the other selecting b then merging in my application.You need to $group our documents and use the $push accumulator operator to return an array of "a" and "b" within the collection.
In the $project operator you use the $setUnion operator to filter out the duplicates.
db.coll.aggregate(
[
{ "$group": {
"_id": null,
"a": { "$push": "$a" },
"b": { "$push": "$b" }
}},
{ "$project": {
"_id": 0,
"merged": { "$setUnion": [ "$a", "$b" ] }
}}
]
)
which produces:
{ "merged" : [ 3, 2, 0, 1 ] }
You could use something like that
db.getCollection('yourdocs').aggregate([
{
$project: {
values: [ "$a", "$b" ]
}
},
{
$unwind: "$values"
},
{
$group: {
_id: "distinctValues",
values: {
$addToSet: "$values"
}
}
}
])
which will result in
{
"_id" : "distinctValues",
"values" : [
0,
1,
2,
3
]
}
Update: Just read you want it sorted which is not guaranteed with $addToSet. If sorting is crucial for you just add the following to the aggregation pipeline after the stages above
{
"$unwind": "$values"
},
{
"$sort": {
"values": 1
}
},
{
"$group": {
_id: "distinctValues",
"values": {
"$push": "$values"
}
}
}
