MongoDB 聚合在根中生成内部数组

Question

您好，我有两个集合 person 和 department。这些集合具有以下属性。

我想聚合一个内部数组并将其添加到文档的根目录中。请检查下面我需要的结果：

人：

{
  _id:ObjectID("5ff93b43535bera64de4"),
  first_name: some name,
  last_name : some name,
  dept: department1,

}

部门：

{
  dept_name: department1,
  dept_descp : this is description,
  books: [
          {
           book_name: book1,
           subject: subject1,
           },
            {
           book_name: book2,
           subject: subject2,
           },
         ]
}

到目前为止我已经尝试过但没有得到所需的结果

person.aggregate([
   {
   $match:
        {
            "_id": ObjectId("5ff93b43535bera64de4")
        }
    },
   {
        $lookup: {
            from: "department",
            localField: "dept",
            foreignField: "dept_name",
            as: "ndept"
        }
   },
   {
     $project:{"books": '$ndept.books'}
   }
   ])

我想得到的结果是

结果：

{
  _id:ObjectID("5ff93b43535bera64de4"),
  first_name: some name,
  last_name : some name,
  dept: department1,
  books: [
          {
           book_name: book1,
           subject: subject1,
           },
            {
           book_name: book2,
           subject: subject2,
           }
         ]
}

Answer 1

尝试 $arrayElemAt 到 select 数组中的第一个元素

{
  $addFields: {
    books: { $arrayElemAt: ["$ndept.books", 0] }
  }
}

Playground