猫鼬加入列表

Question

我是猫鼬的新手，我有两个架构如下

contentschema
{ "_id" : autogenerated
  "title": "string",
  "description": "string",
}

viewedschema
{ "_id" : autogenerated
  "contentid": "ref content",
  "viewedby": "string",
 }

所有查看过内容的用户都将存储在viewedschema集合中，该集合具有contentid引用。

注意：由于查看的记录数量会很大，我不想将内容中的查看作为嵌入文档。

在 Mongoose 中，有没有办法查看所有内容（内容架构数组）。 [类似于 SQL 中的内连接]。

提前致谢。

Answer 1

我们可以使用$lookup 来合并同一数据库的两个不同集合中的文档，并对集合执行左外连接。

让我们下面的文件在content集合

{
        "_id" : ObjectId("59ef51f106b0505f997f84c8"),
        "title" : "myfavoritesong",
        "description" : "A wonderful composition using string instruments"
}
{
        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),
        "title" : "myfavoritestory",
        "description" : "An interesting short story with a twisted ending"
}

viewed 集合中的文档

{
        "_id" : ObjectId("59ef523706b0505f997f84c9"),
        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),
        "viewedby" : "user1"
}
{
        "_id" : ObjectId("59ef52f406b0505f997f84cb"),
        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
        "viewedby" : "user2"
}
{
        "_id" : ObjectId("59ef53c706b0505f997f84cc"),
        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
        "viewedby" : "user3"
}

通过组合两个集合使用 $lookup 的最终聚合查询是

db.viewed.aggregate({
    $lookup:{
        from : "content", 
        localField: "contentid", 
        foreignField:"_id",
        as:"viewed_contents"
    }
})

我们的样本数据的聚合查询结果是

{
        "_id" : ObjectId("59ef523706b0505f997f84c9"),
        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),
        "viewedby" : "user1",
        "viewed_contents" : [
                {
                        "_id" : ObjectId("59ef51f106b0505f997f84c8"),
                        "title" : "myfavoritesong",
                        "description" : "A wonderful composition using string in
struments"
                }
        ]
}
{
        "_id" : ObjectId("59ef52f406b0505f997f84cb"),
        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
        "viewedby" : "user2",
        "viewed_contents" : [
                {
                        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),
                        "title" : "myfavoritestory",
                        "description" : "An interesting short story with a twist
ed ending"
                }
        ]
}
{
        "_id" : ObjectId("59ef53c706b0505f997f84cc"),
        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
        "viewedby" : "user3",
        "viewed_contents" : [
                {
                        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),
                        "title" : "myfavoritestory",
                        "description" : "An interesting short story with a twist
ed ending"
                }
        ]
}

请注意，您还可以将 viewed 中的集合交换为外部集合，将 content 集合为本地集合

db.content.aggregate({
   $lookup:{
     from : "viewed", 
     localField: "_id", 
     foreignField:"contentid",
     as:"contents_viewed_by"
   }
})

本次聚合查询结果如下

{
        "_id" : ObjectId("59ef51f106b0505f997f84c8"),
        "title" : "myfavoritesong",
        "description" : "A wonderful composition using string instruments",
        "contents_viewed_by" : [
                {
                        "_id" : ObjectId("59ef523706b0505f997f84c9"),
                        "contentid" : ObjectId("59ef51f106b0505f997f84c8"),
                        "viewedby" : "user1"
                }
        ]
}
{
        "_id" : ObjectId("59ef52ad06b0505f997f84ca"),
        "title" : "myfavoritestory",
        "description" : "An interesting short story with a twisted ending",
        "contents_viewed_by" : [
                {
                        "_id" : ObjectId("59ef52f406b0505f997f84cb"),
                        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
                        "viewedby" : "user2"
                },
                {
                        "_id" : ObjectId("59ef53c706b0505f997f84cc"),
                        "contentid" : ObjectId("59ef52ad06b0505f997f84ca"),
                        "viewedby" : "user3"
                }
        ]
}