如何在猫鼬的子文档数组中查找字段？

Question

我有一组这样的评论对象：

   "reviews": {
        "author": "5e9167c5303a530023bcae42",
        "rate": 5,
        "spoiler": false,
        "content": "This is a comment This is a comment This is a comment.",
        "createdAt": "2020-04-12T16:08:34.966Z",
        "updatedAt": "2020-04-12T16:08:34.966Z"
    },

我想要实现的是查找 author 字段并获取用户数据，但问题是我尝试使用的查找仅 returns这对我来说：

代码：

 .lookup({
    from: 'users',
    localField: 'reviews.author',
    foreignField: '_id',
    as: 'reviews.author',
  })

回复：

有什么方法可以获取作者在该领域的数据？那里是作者的id。

Answer 1

你应该在对服务器的请求上使用 mongoose 的 populate('author') 方法，该方法获取该作者的 id 并将用户数据添加到 mongoose 的响应中并且不要忘记以连接这两个集合的方式设置架构在您的审核模式中，您应该将 ref 添加到保存作者用户的模式作者：{ 类型：Schema.Types.ObjectId，参考：'users' }，

Answer 2

尝试在您的数据库上执行以下查询：

db.reviews.aggregate([
  /** unwind in general is not needed for `$lookup` for if you wanted to match lookup result with specific elem in array is needed */
  {
    $unwind: { path: "$reviews", preserveNullAndEmptyArrays: true },
  },
  {
    $lookup: {
      from: "users",
      localField: "reviews.author",
      foreignField: "_id",
      as: "author", // Pull lookup result into 'author' field
    },
  },
  /** Update 'reviews.author' field in 'reviews' object by checking if   'author' field got a match from 'users' collection.
   * If Yes - As lookup returns an array get first elem & assign(As there will be only one element returned -uniques),
   * If No - keep 'reviews.author' as is */
  {
    $addFields: {
      "reviews.author": {
        $cond: [
          { $ne: ["$author", []] },
          { $arrayElemAt: ["$author", 0] },
          "$reviews.author",
        ],
      },
    },
  },
  /** Group back the documents based on '_id' field & push back all individual 'reviews' objects to 'reviews' array */
  {
    $group: {
      _id: "$_id",
      reviews: { $push: "$reviews" },
    },
  },
]);

测试：MongoDB-Playground

注意： 以防万一文档中有其他字段以及需要在输出中保留的 reviews 然后从 $group 开始使用这些阶段：

  {
    $group: {
      _id: "$_id",
      data: {
        $first: "$$ROOT"
      },
      reviews: {
        $push: "$reviews"
      }
    }
  },
  {
    $addFields: {
      "data.reviews": "$reviews"
    }
  },
  {
    $project: {
      "data.author": 0
    }
  },
  {
    $replaceRoot: {
      newRoot: "$data"
    }
  }

测试： MongoDB-Playground

注意： 尝试在较小的数据集上保持对运行的查询，可能通过添加 $match 作为第一阶段来过滤文档并具有适当的索引。

Answer 3

你可以按照这个代码

$lookup:{
            from:'users',
            localField:'reviews.author',
            foreignField:'_id',
            as:'reviews.author'
        }
**OR**

> When You find the doc then use populate
> reviews.find().populate("author")

如何在猫鼬的子文档数组中查找字段？

How to lookup a field in an array of subdocuments in mongoose?

mongoose

mongodb

mongodb-query

aggregation-framework

mongoose-populate