现有两个集合如下：
company: {_id: ObjectId("..."), countryId: "566eb3c704c7b31facbb0007"}
country: { _id: ObjectId("566eb3c704c7b31facbb0007"), name:"..."}
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正常关联：
{ $lookup: {
from: "country",
localField: "countryId", <= string
foreignField: "_id", <= ObjectId
as: "country"
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但结果却是无效
解决方案：
使用
$project
db.company.aggregate([
"$project": {
"countryId": {
"$toObjectId": "$countryId"
"$lookup": {
"from": "country",
"localField": "countryId",
"foreignField": "_id",
"as": "country"
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或：
db.company.aggregate([
"$project": {
"_id": {
"$toString": "$_id"
"$lookup": {
"from": "country",
"localField": "countryId",
"foreignField": "_id",
"as": "country"
使用 $lookup 的 let 属性
db.company.aggregate([
"$lookup": {
"let": { "countryId": { "$toObjectId": "$countryId" } },
"from": "country",
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$_id", "?countryId" ] } } }
"as": "country"
使用 $addFields
db.role.aggregate([
{ "$addFields": { "countryId": { "$toObjectId": "$countryId" }}},
{ "$lookup": {
"from": "country",
"localField": "countryId",
"foreignField": "_id",
"as": "country"
- 1504
-
崔庆才丨静觅
MongoDB
Python
- 8.4w
-
whynotgonow
MongoDB
JavaScript
