MongoDB聚合-如何“$lookup”嵌套文档“_id”？

Question

多亏了这里的人的帮助，我成功地在我的文档中找到了两个ID，并在另一个集合中找到了他们的代表文档。我需要采取的下一步是进一步查找一个“嵌套”ID（引用另一个集合中的文档）。

db.getCollection('village').aggregate([
{
    "$match": { _id: "111" }
},
{
    "$lookup": {
        from: "character",
        localField: "chieftainId",
        foreignField: "_id",
        as: "chieftain"
    } 
},
{
     "$lookup": {
        from: "character",
        localField: "villagerIds",
        foreignField: "_id",
        as: "villagers"
    }
},
{
     "$lookup": {
        from: "bloodline",
        localField: "chieftain.bloodline",
        foreignField: "_id",
        as: "chieftain.bloodline"
    }
},
 { "$project" : { "villagerIds" : 0, "chieftainId" : 0}},
 { "$unwind" : "$chieftain" }
])

其结果如下：

{
    "_id" : "111",
    "name" : "MyVillage",
    "reputation" : 0,
    "chieftain" : {
        "bloodline" : []
    },
    "villagers" : [ 
        {
            "_id" : "333",
            "name" : "Bortan",
            "age" : 21,
            "bloodlineId" : "7f02191f-90af-406e-87ff-41d5b4387999",
            "villageId" : "foovillage",
            "professionId" : "02cbb10a-6c0f-4249-a932-3f40e12d32c5"
        }, 
        {
            "_id" : "444",
            "name" : "Blendi",
            "age" : 21,
            "bloodlineId" : "b3a8ffeb-27aa-4e2e-a8e6-b382554f326a",
            "villageId" : "foovillage",
            "professionId" : "45dc9350-c84a-491d-a49a-524834dd5773"
        }
    ]
}

我希望chieftain部分是这样的（这就是没有添加'last'$lookup'的chieftain文档的样子）：

"chieftain" : {
        "_id" : "222",
        "name" : "Bolzan",
        "age" : 21,
        "bloodlineId" : "7c2926f9-2f20-4ccf-846a-c9966970fa9b", // this should be resolved/lookedup
        "villageId" : "foovillage",
    },

Answer 1

在查找时，chieftan是一个数组，因此设置chieftan.bloodline将用一个只包含bloodline字段的对象替换数组。

将{“$unwind”：“$chieftain”}阶段移动到血统查找阶段之前，以便查找处理对象。