After some aggregations I have only one document like this:
{
a:[3,5,8,1,...],
b:[5,2,6,5,...],
c:[2,6,3,1,...]
}
What is the best way to get:
{
a:[1,2,3,4,...],
b:[1,2,3,4,...],
c:[1,2,3,4,...]
}
I need it to be sorted to do other aggregations so I can't use find or update.
The closest working solution I've got is the following aggregation:
[
{
$facet: {
a: [
{ $unwind: '$a' },
{ $sort: { a: 1 } },
{
$group: {
_id: '$_id',
a: { $push: '$a' }
}
}
],
b: [
{ $unwind: '$b' },
{ $sort: { b: 1 } },
{
$group: {
_id: '$_id',
b: { $push: '$b' }
}
}
],
c: [
{ $unwind: '$c' },
{ $sort: { c: 1 } },
{
$group: {
_id: '$_id',
c: { $push: '$c' }
}
}
]
}
},
{
$set: {
a: { $arrayElemAt: ['$a.a', 0] },
b: { $arrayElemAt: ['$b.b', 0] },
c: { $arrayElemAt: ['$c.c', 0] }
}
}
]
In short, what this does is: $facet performs identical operations on each a, b, c field in parallel; each field is split with $unwind, sorted in ascending order and then grouped on itself.
Until this point we already have sorted fields but each field is now an object with appropriate name (for example, { _id: '', a: [...] }). So, to return to the array we had earlier, another stage is to simply extract only the needed field from each object. This is done with $set and $arrayElemAt (which takes first array field).
So, in the end we have each field sorted in ascending order while having the same amount of documents.
Note: this only works if you have a single result document. If you have multiple of them, then you will need to slightly change stages:
$facet stage remains the same$set (second) stage is changed to:Use $unwind + updated $set stages for each separate field (in this case, we have a, b, c):
{ $unwind: '$a' },
{ $unwind: '$b' },
{ $unwind: '$c' },
{ $set: { a: '$a.a' } },
{ $set: { b: '$b.b' } },
{ $set: { c: '$c.c' } }
This might not be the prettiest approach but I could not find shorter version.
$facet operator, I was doing the same but not in parallel so it's really helpful !
Starting at version 4.4, you can use $function and sort your arrays directly with javascript.
db.collection.aggregate([
{
$set: {
"a": {
$function: {
body: "function(a){a.sort((x, y)=>x-y);return a;}",
args: [
"$a"
],
lang: "js"
}
},
"b": {
$function: {
body: "function(b){b.sort((x, y)=>x-y);return b;}",
args: [
"$b"
],
lang: "js"
}
},
"c": {
$function: {
body: "function(c){c.sort((x, y)=>x-y);return c;}",
args: [
"$c"
],
lang: "js"
}
}
}
}
])
