MongoDB从数组返回内部文档

Question

我正在尝试从文档中的数组中获取元素，并且只获取我不想要整个文档的元素

我尝试了不同的方法，但它们都返回了整个文档

db.dept.find({"section.classes.CRN":"1901"}).limit(100)
db.dept.where("section.classes.CRN").eq("1901").limit(100)
json
{
"_id" : ObjectId("5d70ab0c280d6b8ebb850cc1"),
"name" : "Art Studio",
"abbr" : "ARS",
"section" : [
{
"type" : "Undergraduate Courses",
"classes" : [
{
"CRN" : "193",
"Course" : "ARS100",
"Sec" : "01",
"Title" : "Drawing I",
"Cr" : "3",
"Dates" : "8/26-12/19",
"Days" : "MR",
"Time" : "1230P-0320P",
"Loc" : "SAB 226",
"Instructor" : "Schuck",
"Attributes" : "",
"Avail" : "F"
},
{
"CRN" : "293",
"Course" : "ARS100",
"Sec" : "02",
"Title" : "Drawing I",
"Cr" : "3",
"Dates" : "8/26-12/19",
"Days" : "MR",
"Time" : "0330P-0620P",
"Loc" : "SAB 226",
"Instructor" : "Itty",
"Attributes" : "",
"Avail" : "F"
},
{...

我在搜索一组 CRN 值时试图获得这个或类似的东西

json
[  {
"CRN" : "193",
"Course" : "ARS100",
"Sec" : "01",
"Title" : "Drawing I",
"Cr" : "3",
"Dates" : "8/26-12/19",
...
"Instructor" : "Schuck",
"Attributes" : "",
"Avail" : "F"
}
]

Answer 1

尝试使用聚合管道将双嵌套数组投影为：

输入：

[
{
"_id": ObjectId("5d70ab0c280d6b8ebb850cc1"),
"name": "Art Studio",
"abbr": "ARS",
"section": [
{
"type": "Undergraduate Courses",
"classes": [
{
"CRN": "193",
"Course": "ARS100",
"Sec": "01",
"Title": "Drawing I",
"Cr": "3",
"Dates": "8/26-12/19",
"Days": "MR",
"Time": "1230P-0320P",
"Loc": "SAB 226",
"Instructor": "Schuck",
"Attributes": "",
"Avail": "F"
},
{
"CRN": "293",
"Course": "ARS100",
"Sec": "02",
"Title": "Drawing I",
"Cr": "3",
"Dates": "8/26-12/19",
"Days": "MR",
"Time": "0330P-0620P",
"Loc": "SAB 226",
"Instructor": "Itty",
"Attributes": "",
"Avail": "F"
}
]
}
]
}
]

查询： 在下面的展开部分，您可以过滤 CRN 的类

db.collection.aggregate([
{
$unwind: "$section"
},
{
$project: {
name: 1,
abbr: 1,
"section.type": 1,
"section.classes": {
$filter: {
input: "$section.classes",
as: "item",
cond: {
$eq: [
"$$item.CRN",
"193"
]
}
}
}
}
},
{
$group: {
_id: "$_id",
section: {
$push: "$section"
}
}
}
])

输出： 可以在 Project 中根据需要管理密钥，以添加新密钥或替换密钥。

[
{
"_id": ObjectId("5d70ab0c280d6b8ebb850cc1"),
"section": [
{
"classes": [
{
"Attributes": "",
"Avail": "F",
"CRN": "193",
"Course": "ARS100",
"Cr": "3",
"Dates": "8/26-12/19",
"Days": "MR",
"Instructor": "Schuck",
"Loc": "SAB 226",
"Sec": "01",
"Time": "1230P-0320P",
"Title": "Drawing I"
}
],
"type": "Undergraduate Courses"
}
]
}
]

Answer 2

db.dept.find({"section.classes.CRN":"1901"},{"section.classes":1}).limit(100)

它在 mongodb 中称为projection，您在查找查询中传递第二个对象以指定您想要在结果中的字段。

因此，根据上述情况，如果您想要名称和结果中的部分，您应该传递这样的东西

db.dept.find({"section.classes.CRN":"1901"},{"name":1, "section":1}).limit(100)