将$ lookup结果转换为对象而不是数组

Question

我正在从_id进行$ lookup.所以结果总是1个文件.因此,我希望结果是一个对象,而不是一个包含一个项目的数组.

let query = mongoose.model('Discipline').aggregate([
    {
      $match: {
        project: mongoose.Types.ObjectId(req.params.projectId)
      },
    },
    {
      $lookup: {
        from: "typecategories",
        localField: "typeCategory",
        foreignField: "_id",
        as: "typeCategory"
      }
    },
    {
      $project: {
        title: 1, typeCategory: "$typeCategory[0]"
      }
    }
  ]);

这种表示法:"$typeCategory[0]"不起作用.这有什么聪明的方法吗？

Answer 1

你可以使用$unwind.它从输入文档中解构数组字段,以输出每个元素的文档

let query = mongoose.model('Discipline').aggregate([
    {
      $match: {
        project: mongoose.Types.ObjectId(req.params.projectId)
      },
    },
    {
      $lookup: {
        from: "typecategories",
        localField: "typeCategory",
        foreignField: "_id",
        as: "typeCategory"
      }
    },
    {$unwind: '$typeCategory'},
    {
      $project: {
        title: 1, typeCategory: "$typeCategory"
      }
    }
  ]);

Answer 2

您可以在stage 中使用$arrayElemAt$project。

$arrayElemAtis的语法{ $arrayElemAt: [ <array>, <idxexOfArray> ] }

喜欢：

mongoose.model('Discipline').aggregate([
   {
      $match: {
        project: mongoose.Types.ObjectId(req.params.projectId)
      },
   },
   {
      $lookup: {
        from: "typecategories",
        localField: "typeCategory",
        foreignField: "_id",
        as: "typeCategory"
      }
   },
   {
      $project: {
         name: 1, typeCategory: {$arrayElemAt:["$typeCategory",0]}
      }
   }
]);