$slice允許我得到一個嵌套陣列的切片。我像這樣成功使用它：
const user = await User.aggregate([
{ $match: { _id: ObjectId(user_id) } },
{
$lookup: {
from: "users",
let: { friends: "$friends" },
pipeline: [
{ $match: { $expr: { $in: ["$_id", "$$friends"] } } },
{
$lookup: {
from: "profiles",
localField: "profile",
foreignField: "_id",
as: "profile",
},
},
{
$match: {
"profile.online": true,
},
},
{
$project: {
name: "$name",
surname: "$surname",
profile: { $arrayElemAt: ["$profile", 0] },
},
},
],
as: "friends",
},
},
{
$addFields: {
friends: {
$slice: ["$friends", skip, limit],
},
},
},
]);
現在，我不想取切片，而是取陣列 field 的隨機樣本friends。
我找不到辦法做到這一點。但是，在小組賽階段我可以使用這樣的東西：
const pipeline = [
{
$lookup: {
from: "profiles",
let: { profiles_id: "$profile" },
pipeline: [
{
$match: {
online: true,
$expr: { $eq: ["$_id", "$$profiles_id"] },
},
},
],
as: "profile",
},
},
{ $unwind: "$profile" },
{ $sample: { size: 10 } },
];
const users = await User.aggregate(pipeline);
uj5u.com熱心網友回復：
將最后$addFields階段更改為此。
優點：它“有效”。
缺點：不能保證串列中的隨機條目是唯一的。要做到這一點需要做更多的作業。如果你的朋友比范圍多，那么你可能沒問題。
,{$addFields: {friends: {$reduce: { // overwrite friends array...
// $range is the number of things you want to pick:
input: {$range:[0,4]},
initialValue: [],
in: {
$let: {
// qq will be a random # between 0 and size-1 thanks to mult
// and floor, so we do not have to do qq-1 to get to zero-based
// indexing on the $friends array
vars: {qq: {$floor:{$multiply:[{$rand: {}},{$size:"$friends"}]}} },
// $concat only works for strings, but $concatArrays can be used
// (creatively) on other types. Here $slice returns an array of
// 1 item which we easily pass to $concatArrays to build the
// the overall result:
in: {$concatArrays: [ "$$value", {$slice:["$friends","$$qq",1]} ]}
}}
}}
更新
此版本利用在$reduce鏈中保持狀態并且不會選擇欺騙。它通過在隨機選擇每個專案時迭代地縮小專案的輸入候選串列來做到這一點。輸出有點嵌套（即friends未設定為選取的隨機樣本，而是設定為包含選取和剩余 aa 串列的物件），但這很容易在事后重新格式化。在 MongoDB 5.0 中，我們可以通過以下方式完成它：
{$addFields: {friends: {$getField: {field: "$friends.picks", input: {$reduce: {
但是很多人還沒有上5.0。
{$addFields: {friends: {$reduce: {
// $range is the number of things you want to pick:
input: {$range:[0,6]},
// This is classic use of $reduce to iterate over something AND
// preserve state. We start with picks as empty and aa being the
// original friends array:
initialValue: {aa: "$friends", picks: []},
in: {
$let: {
// idx will be a random # between 0 and size-1 thanks to mult
// and floor, so we do not have to do idx-1 to get to zero-based
// indexing on the $friends array. idx and sz will be eval'd
// each time reduce turns the crank through the input range:
vars: {idx: {$floor:{$multiply:[{$rand: {}},{$size:"$$value.aa"}]}},
// cannot set sz and then use it in same vars; oh well
sz: {$size:"$$value.aa"}
},
in: {
// Add to our picks list:
picks: {$concatArrays: [ "$$value.picks", {$slice:["$$value.aa","$$idx",1]} ]},
// And now shrink up the input candidate array.
// Sadly, we cannot do $slice:[array,pos,0] to yield an empty
// array and keep the $concat logic tight; thus we have to test
// for front and end special conditions.
// This whole bit is to extract the chosen item from the aa
// array by splicing together a new one MINUS the target.
// This will change the value of $sz (-1) as we crank thru
// the picks. This ensures we only pick UNPICKED items from
// $$value.aa!
aa: {$cond: [{$eq:["$$idx",0]}, // if
// idx 0: Take from idx 1 and count size - 1:
{$slice:["$$value.aa",1,{$subtract:["$$sz",1]}]}, // then
// idx last: Take from idx 0 and ALSO count size - 1:
{$cond: [ // else
{$eq:["$$idx",{$subtract:["$$sz",1]}]}, // if
{$slice:["$$value.aa",0,{$subtract:["$$sz",1]}]}, // then
// else not 0 or last item, i.e. idx = 3
{$concatArrays: [
// Start at 0, count idx; this will land
// us BEFORE the target item (because idx
// is n-1:
{$slice:["$$value.aa",0,"$$idx"]},
// Jump over the target ( 1), and go n-2
// (1 for idx/n conversion, and 1 for the
// fact we jumped over:
{$slice:["$$value.aa",{$add:["$$idx",1]},{$subtract:["$$sz",2]}]}
]}
]}
]}
}
}}
}}
}}
]);
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