英文:
Mongo sort by either of 2 fields
问题
我有一些文件，其中foo字段始终嵌套在x或y中，而不是同时出现。
我想要按此字段排序并获取结果。
如果可以使用find().sort()来实现，那将很好，但如果需要使用aggregate()，也希望得到相关答案。
英文:
I have documents where the foo field is always nested in x or y and not both.
{
"_id": ObjectId("5a6eb97b2937803b14255180"),
"x": {
"foo": 42
},
"y": { }
},
{
"_id": ObjectId("5a6eb97b2937803b14255181"),
"x": { },
"y": {
"foo": 24
}
}
I want to sort by it and get
{
"_id": ObjectId("5a6eb97b2937803b14255181"),
"x": { },
"y": {
"foo": 24
}
},
{
"_id": ObjectId("5a6eb97b2937803b14255180"),
"x": {
"foo": 42
},
"y": { }
}
It would be great if I can do it with find().sort() and not aggregate(), but an aggregate() answer would be appriciated too.
答案1
得分: 1
使用$ifNull创建一个排序键，如果x.foo不存在，则回退到y.foo，然后根据排序键进行排序。
db.collection.aggregate([
{
"$set": {
"sortKey": {
"$ifNull": [
"$x.foo",
"$y.foo"
]
}
}
},
{
$sort: {
sortKey: 1
}
},
{
"$unset": "sortKey"
}
])
英文:
Create a sort key with $ifNull to fall back into y.foo if x.foo is not present then sort on the sort key.
db.collection.aggregate([
{
"$set": {
"sortKey": {
"$ifNull": [
"$x.foo",
"$y.foo"
]
}
}
},
{
$sort: {
sortKey: 1
}
},
{
"$unset": "sortKey"
}
])
Side note: It might be better if you can preprocess your documents and materialized a single sort key. You can then index the consolidated sort key to gain performance boost and simpler queries.
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