在mongodb中相对较新，我对这三个表执行了聚合管道，并且令人惊讶的是，在管道的第三个查找阶段，我将“ r2”数组设置为空。我对字段名称进行了交叉检查，一切正常。
db.user.aggregate([
{
$lookup:{
from:"enrollment",
localField:"user_id",
foreignField:"userID",
as:"r1"
}
},
{
$unwind:{
path:"$r1",
includeArrayIndex:"r1_id"
}
},
{
$lookup:{
from:"course",
localField:"r1.courseID",
foreignField:"courseID",
as:"r2"
}
}
])
我作为用户，注册和课程有三个集合。
用户身份
> db.user.find()
{ "_id" : ObjectId("5ef4ba8d500ac8876da0d2ca"), "user_id" : 1, "first_name" : "Christian",
"last_name" : "Hur", "email" : "[email protected]", "password" : "abc1234" }
{ "_id" : ObjectId("5ef4ba8d500ac8876da0d2cb"), "user_id" : 2, "first_name" : "Mary", "last_name" :
"Jane", "email" : "[email protected]", "password" : "password123" }
{ "_id" : ObjectId("5ef4bc2563742adee5403b1d"), "user_id" : 3, "first_name" : "ari", "last_name" :
"dutta", "email" : "[email protected]", "password" : "po1234" }
并且，当然
> db.course.find()
{ "_id" : ObjectId("5ef4c1b64a77aec0af5e73ae"), "courseID" : 3333, "title" : "Adv PHP 201",
"description" : "Advance PHP programming", "credits" : 3, "term" : "fall" }
{ "_id" : ObjectId("5ef4c20d4a77aec0af5e73af"), "courseID" : 5555, "title" : "Java 201",
"description" : "Advanced Programming", "credits" : 4, "term" : "fall"}
{ "_id" : ObjectId("5ef4c2564a77aec0af5e73b0"), "courseID" : 6666, "title" : "Angular 1",
"description" : "Intro to Angular", "credits" : 3, "term" : "fall,spring" }
和报名一样
> db.enrollment.find()
{ "_id" : ObjectId("5ef771f42d98ffab4460a651"), "userID" : 1, "courseID" : "3333" }
{ "_id" : ObjectId("5ef7722d2d98ffab4460a652"), "userID" : 1, "courseID" : "6666" }
结果
{ "_id" : ObjectId("5ef4ba8d500ac8876da0d2ca"), "user_id" : 1, "first_name" :
"Christian", "last_name" : "Hur", "email" : "[email protected]", "password" :
"abc1234", "r1" : { "_id" : ObjectId("5ef771f42d98ffab4460a651"), "userID" :
1, "courseID" : "3333" }, "r1_id" : NumberLong(0), "r2" : [ ] }
{ "_id" : ObjectId("5ef4ba8d500ac8876da0d2ca"), "user_id" : 1, "first_name" :
"Christian", "last_name" : "Hur", "email" : "[email protected]", "password" :
"abc1234", "r1" : { "_id" : ObjectId("5ef7722d2d98ffab4460a652"), "userID" :
1, "courseID" : "6666" }, "r1_id" : NumberLong(1), "r2" : [ ] }
我也检查了文档，但没有帮助，如何解决？
你有一种不对齐courseID的集之间，在enrollment它的类型string和course它的类型number。
更改$lookup为：
{
$lookup: {
from: "course",
let: {
courseID: {
$toInt: "$r1.courseID"
}
},
pipeline: [
{
$match: {
$expr: {
$eq: [
"$$courseID",
"$courseID"
]
}
}
}
],
as: "r2"
}
}
自己尝试：Mongo Playground
另外，由于您说的是新手，因此我个人建议您使用该_id字段，而不要使用自己生成的courseID/ user_id。它只会使维护更容易。
对于Mongo 4.2+版本，以下是如何更新注册收集字段：
db.enrollment.updateMany(
{},
[
{
$set: {
courseID: {$toInt: "$courseID"}
}
}
]
)
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