Mongo Lookup with nested DBRefs

Question

我在MongoDB中有一个名为"post"的集合，其中包含指向"author"的DBRef列表，而DBRef又将DBRef发送到集合"media"。

当我检索帖子时，我想获取帖子的整个对象，包括对象中的所有作者和媒体。

例如，这是我的收藏：

post:
{
"_id" : ObjectId("5d7db7af49c5e277395871bd"),
"authors" : [
DBRef("author", ObjectId("5d7daaab49c5e277395871ba")),
DBRef("author", ObjectId("5d7daaab49c5e277395871bb"))
]
}
author:
[{
"_id" : ObjectId("5d7daaab49c5e277395871ba"),
"name" : "author 1",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b8"))
},
{
"_id" : ObjectId("5d7daaab49c5e277395871bb"),
"name" : "author 2",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b9"))
}]
media:
[{
"_id" : ObjectId("5d7daa2049c5e277395871b8"),
"url" : "http://image.com/1",
"type" : "png"
},
{
"_id" : ObjectId("5d7daa2049c5e277395871b9"),
"url" : "http://image.com/2",
"type" : "png"
}]

我希望查询有以下结果：

post:
{
"_id" : ObjectId("5d7db7af49c5e277395871bd"),
"authors" : [
{
"_id" : ObjectId("5d7daaab49c5e277395871ba"),
"name" : "author 1",
"media" : {
"_id" : ObjectId("5d7daaab49c5e277395871ba"),
"name" : "author 1",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b8"))
}
},
{
"_id" : ObjectId("5d7daaab49c5e277395871bb"),
"name" : "author 2",
"media" : {
"_id" : ObjectId("5d7daaab49c5e277395871bb"),
"name" : "author 2",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b9"))
}
}
]
}

到目前为止，我有以下查询，其中包括帖子中的作者，但我也需要帮助来检索作者中的媒体。

current query:
db.post.aggregate([
{
$project: { 
authors: {
$map: { 
input: { 
$map: {
input:"$authors",
in: {
$arrayElemAt: [{$objectToArray: "$$this"}, 1]
},
}
},
in: "$$this.v"}},
}
}, 
{
$lookup: {
from:"author", 
localField:"authors",
foreignField:"_id", 
as:"authors"
}
},
])
current result:
{
"_id" : ObjectId("5d7db7af49c5e277395871bd"),
"authors" : [
{
"_id" : ObjectId("5d7daaab49c5e277395871ba"),
"name" : "author 1",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b8"))
},
{
"_id" : ObjectId("5d7daaab49c5e277395871bb"),
"name" : "author 2",
"media" : DBRef("media", ObjectId("5d7daa2049c5e277395871b9"))
}
]
}

Answer 1

尝试使用$unwind，然后按 id$goup

你可以这样做

db.post.aggregate([{
$project: {
authors: {
$map: {
input: {
$map: {
input: "$authors",
in: {
$arrayElemAt: [{
$objectToArray: "$$this"
}, 1]
},
}
},
in: "$$this.v"
}
},
}
}, {
$lookup: {
from: "author",
localField: "authors",
foreignField: "_id",
as: "authors"
}
}, {
$unwind: "$authors"
}, {
$project: {
"authors.media": {
$arrayElemAt: [{
$objectToArray: "$authors.media"
}, 1]
},
"authors._id": 1,
"authors.name": 1
}
}, {
$lookup: {
from: "media",
localField: "authors.media.v",
foreignField: "_id",
as: "authors.media"
}
}, {
$unwind: {
path: "$authors.media",
preserveNullAndEmptyArrays: true
}
}, {
$group: {
_id: "$_id",
authors: {
$push: "$authors"
}
}
}]).pretty()

这是结果

"_id" : ObjectId("5d7db7af49c5e277395871bd"),
"authors" : [
{
"_id" : ObjectId("5d7daaab49c5e277395871ba"),
"name" : "author 1",
"media" : {
"_id" : ObjectId("5d7daa2049c5e277395871b8"),
"url" : "http://image.com/1",
"type" : "png"
}
},
{
"_id" : ObjectId("5d7daaab49c5e277395871bb"),
"name" : "author 2",
"media" : {
"_id" : ObjectId("5d7daa2049c5e277395871b9"),
"url" : "http://image.com/2",
"type" : "png"
}
}
]
}

您可以在中间阶段使用$project更改格式

$unwind可以使用选项。以下$unwind操作使用 preserveNullAndEmptyArrays 选项在输出中包含缺少 size(null 或空数组(的文档。

[
{ $unwind: { path: "$sizes", preserveNullAndEmptyArrays: true } }
]

这是文件