Use $size with $sort in array and sub array - mongodb

Question

Here's the structure part of my collection:
_id: ObjectId("W"),
names: [
{
number: 1,
subnames: [ { id: "X", day: 1 }, { id: "Y", day: 10 }, { id: "Z", day: 2 } ],
list: ["A","B","C"],
day: 1
},
{
number: 2,
day: 5
},
{
number: 3,
subnames: [ { id: "X", day: 8 }, { id: "Z", day: 5 } ],
list: ["A","C"],
day: 2
},
...
],
...
I use this request:
db.publication.aggregate( [ { $match: { _id: ObjectId("W") } }, { $group: { _id: "$_id", SizeName: { $first: { $size: { $ifNull: [ "$names", [] ] } } }, names: { $first: "$names" } } }, { $unwind: "$names" }, { $sort: { "names.day": 1 } }, { $group: { _id: "$_id", SzNames: { $sum: 1 }, names: { $push: { number: "$names.number", subnames: "$names.subnames", list: "$names.list", SizeList: { $size: { $ifNull: [ "$names.list", [] ] } } } } } } ] );
but I would now use $sort for my names array AND my subnames array to obtain this result (subnames may not exist) :
_id: ObjectId("W"),
names: [
{
number: 2,
SizeList: 0,
day: 5
},
{
number: 3,
subnames: [ { id: "Z", day: 5 }, { id: "X", day: 8 } ],
list: ["A","C"],
SizeList: 2,
day: 2
},
{
number: 1,
subnames: [ { id: "X", day: 1 }, { id: "Z", day: 2 }, { id: "Y", day: 10 } ],
list: ["A","B","C"],
SizeList: 3,
day: 1
}
...
],
...
Can you help me ?

Answer 1

You can do this, but with great difficulty. I for one would gladly vote for an inline version of $sort along the lines of the $map operator. That would makes things so much easier.
For now though you need to de-construct and re-build the arrays after sorting. And you have to be very careful about this. Hence make false arrays with a single entry before processing $unwind:
db.publication.aggregate([
{ "$project": {
"SizeNames": {
"$size": {
"$ifNull": [ "$names", [] ]
}
},
"names": { "$ifNull": [{ "$map": {
"input": "$names",
"as": "el",
"in": {
"SizeList": {
"$size": {
"$ifNull": [ "$$el.list", [] ]
}
},
"SizeSubnames": {
"$size": {
"$ifNull": [ "$$el.subnames", [] ]
}
},
"number": "$$el.number",
"day": "$$el.day",
"subnames": { "$ifNull": [ "$$el.subnames", [0] ] },
"list": "$$el.list"
}
}}, [0] ] }
}},
{ "$unwind": "$names" },
{ "$unwind": "$names.subnames" },
{ "$sort": { "_id": 1, "names.subnames.day": 1 } },
{ "$group": {
"_id": {
"_id": "$_id",
"SizeNames": "$SizeNames",
"names": {
"SizeList": "$names.SizeList",
"SizeSubnames": "$names.SizeSubnames",
"number": "$names.number",
"list": "$names.list",
"day": "$names.day"
}
},
"subnames": { "$push": "$names.subnames" }
}},
{ "$sort": { "_id._id": 1, "_id.names.day": 1 } },
{ "$group": {
"_id": "$_id._id",
"SizeNames": { "$first": "$_id.SizeNames" },
"names": {
"$push": { "$cond": [
{ "$ne": [ "$_id.names.SizeSubnames", 0 ] },
{
"number": "$_id.names.number",
"subnames": "$subnames",
"list": "$_id.names.list",
"SizeList": "$_id.names.SizeList",
"day": "$_id.names.day"
},
{
"number": "$_id.names.number",
"list": "$_id.names.list",
"SizeList": "$_id.names.SizeList",
"day": "$_id.names.day"
}
]}
}
}},
{ "$project": {
"SizeNames": 1,
"names": {
"$cond": [
{ "$ne": [ "$SizeNames", 0 ] },
"$names",
[]
]
}
}}
])
You can kind of "hide away" the original empty array from the inner document as shown, but it's really difficult to remove all presence of the outer "names" array without pulling a similar conditional array "push" technique, and that really isn't a practical approach.
If all of this is just about sorting array elements in individual documents though, the aggregation framework should not be the tool to do this. It can be done as shown, but per document this is much easier to do in client side code.
Output:
{
"_id" : ObjectId("54b5cff8102f292553ce9bb5"),
"SizeNames" : 3,
"names" : [
{
"number" : 1,
"subnames" : [
{
"id" : "X",
"day" : 1
},
{
"id" : "Z",
"day" : 2
},
{
"id" : "Y",
"day" : 10
}
],
"list" : [
"A",
"B",
"C"
],
"SizeList" : 3,
"day" : 1
},
{
"number" : 3,
"subnames" : [
{
"id" : "Z",
"day" : 5
},
{
"id" : "X",
"day" : 8
}
],
"list" : [
"A",
"C"
],
"SizeList" : 2,
"day" : 2
},
{
"number" : 2,
"SizeList" : 0,
"day" : 5
}
]
}