Question 229005 in Mongodb Question posted in
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Mongodb – How to slice certain amount of elements from sub-array in Mongoose?

There’re hundreds of items and each item contains comments. Each comment contains author, content, likes, replies. Since each item is likely to have hundreds of comments, I embedded comment’s id into sub-array inside item document.

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 }
);

When I fetch an item, I am going to load 10 latest comments for that item. On the client side, there is load more button. Every time a user clicks on it, it loads 10 more previous comments. How can I achieve this with mongoose in Express.js?

Tags:

2

Answers


  1. 0 
    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 : [];

    You can skip sort pipeline if createdAt is not present in comment object.

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  2. 0

    If the comments are sorted, you can simply use $slice:

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

    Where pageSize = 10 in your case.

    If the comments are not sorted, you can sort them using a pipeline with $sortArray. With current mongoDB versions there is no reason to $unwind and $group again.

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