如何在Mongoose中从子数组中切片一定数量的元素？

5sxhfpxr 于 2023-10-19 发布在 Go

关注(0)|答案(2)|浏览(113)

有数百个项目，每个项目都包含评论。每个评论包含作者，内容，喜欢，回复。由于每个条目可能有数百条注解，我将注解的id嵌入到条目文档的子数组中。

const itemSchema = new mongoose.Schema(
  {
    name: {
      type: String,
      required: true,
      trim: true,
      index: true,
    },
    author: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "User",
      required: true,
    },
    comments: [{ type: mongoose.Schema.Types.ObjectId, ref: "Comment" }],    
  },
  { timestamps: true }
);

const commentSchema = new mongoose.Schema(
  {
    author: {
      type: mongoose.Schema.Types.ObjectId,
      ref: User,
      required: true,
    },
    content: {
      type: String,
      required: true,
      trim: true,
    },
    likes: {
      type: [{ type: mongoose.Schema.Types.ObjectId, ref: User }],
      default: Array,
    },
    replies: { type: [this], default: Array },
  },
  { timestamps: true }
);

当我获取一个项目时，我将为该项目加载10条最新评论。在客户端，有加载更多按钮。用户每次点击它，它都会加载10个以前的评论。我如何在Express.js中使用mongoose实现这一点呢？

mongoose

来源：https://stackoverflow.com/questions/76892064/how-to-slice-certain-amount-of-elements-from-sub-array-in-mongoose

2条答案

按热度按时间

ljsrvy3e1#

const pageSize = 10; // Number of comments per page
const pageNumber = 1; // Page number (adjust as needed)

const pipeline = [
  // Match the relevant post
  { $match: { _id: ObjectId("your_post_id") } },

  // Project to retrieve only the comments array
  { $project: { _id: 0, comments: 1 } },

  // Unwind the comments array to prepare for sorting
  { $unwind: "$comments" },

  // Sort the comments by creation date in descending order
  { $sort: { "comments.created_at": -1 } },

  // Skip to the appropriate page
  { $skip: (pageNumber - 1) * pageSize },

  // Limit the number of comments per page
  { $limit: pageSize },

  // Group the comments back into an array
  { $group: { _id: null, comments: { $push: "$comments" } } }
];

const result = await db.posts.aggregate(pipeline).toArray();

// Extract the comments array from the result
const sortedComments = result.length > 0 ? result[0].comments : [];

如果comment对象中不存在createdAt，则可以跳过排序管道。

赞(0）回复(0）举报 2023-10-19

sqyvllje2#

如果注解是排序的，你可以简单地使用$slice：

const result = await posts.find(
  { _id: ObjectId("your_post_id")},
  { comments: { $slice: [ pageNumber * pageSize , pageSize  ] } } 
).populate('comments').exec();

其中pageSize = 10在您的情况下。
如果注解没有排序，可以使用管道$sortArray进行排序。对于当前的mongoDB版本，没有理由再次使用$unwind和$group。

赞(0）回复(0）举报 2023-10-19

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如何在Mongoose中从子数组中切片一定数量的元素？

2条答案

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