Question

我有很多这样的评论对象：

   "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，但是问题是我尝试使用的查找仅将其返回给我：

代码：

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

响应：

有什么方法可以获取作者在该字段中的数据？那就是作者的ID。

Answer 1

尝试对您的数据库执行以下查询：

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，如果文档中还有其他字段需要与reviews一起保留在输出中，则从$group开始，请使用以下阶段： / p>

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

测试： MongoDB-Playground

注意：可能要保持查询在较小的数据集上运行，可以通过添加$match作为过滤文档的第一阶段并具有适当的索引。

Answer 2

您应该在对服务器的请求中使用猫鼬的populate('author')方法，该服务器获取该作者的ID并将用户数据添加到猫鼬的响应中并且不要忘记以连接这两个集合的方式来设置架构在您的审阅模式中，应该将引用添加到保存作者用户的模式中作者：{类型：Schema.Types.ObjectId，参考：“用户”}，

Answer 3

您可以遵循以下代码

$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？

3 个答案: