How to query based on related element's field value?

Question

In my database, every article has its own 6-digit identifying number (in addition to id). Every comment belongs to one of the articles. My question is, how to query comments knowing only the article number, but not the article's id?

controllers/fetch-comments.js:

const Article = require('../models/article')
const User = require('../models/user');
const Comment = require('../models/comment');

module.exports = async function (req, res) {
  try {
    let { article_number, articleId } = req.body
    let filter;
    console.log(article_number)
    /* Here is my failed attempt: */
    if (article_number) filter = { 'article.number': article_number };
    if (articleId) filter = { 'article': articleId };
    let projection = null
    let options = null
    let comments = await Comment.find(filter, projection, options).populate('author', 'username fullname').lean()
    return res.status(200).send(comments)
  } catch (error) {
    console.log("error on getting comments: " + error.message);
    return res.status(500).send('Server side error');
  }
}

models/article.js:

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const articleSchema = new Schema({
    author: { type: Schema.Types.ObjectId, ref: 'User', required: true },
    title: { type: String, required: true },
    content: { type: String, required: true },
    coverPhotoUrl: { type: String, required: false },
    number: { type: Number, required: true }, // number is public id for address bar 
    createdAt: { type: Date, required: true },
    category: { type: String, required: false },
    fake: { type: Boolean, required: false },
});

module.exports = mongoose.model('Article', articleSchema);

models/comment.js:

const mongoose = require('mongoose');

const Schema = mongoose.Schema;

const commentSchema = new Schema({
    author: { type: Schema.Types.ObjectId, ref: 'User', required: true },
    content: { type: String, required: true },
    createdAt: { type: Date, required: true },
    article: { type: Schema.Types.ObjectId, ref: 'Article', required: true }
});

module.exports = mongoose.model('Comment', commentSchema);

Please do not suggest doing 2 queries intead of one, I already know how to do that.

Rachit Arora · Accepted Answer · 2021-12-17 07:38:26Z

1

You can use the aggregation framework and use a $match and $lookup operator to get the comment of a particular article number. Following would be the code for the same:

db.Article.aggregate( [


{ $match : { number : article_number} },

{
     $lookup:
   {
     from: "Comment",
     localField: "_id",
     foreignField: "article",
     as: "article_comments"
   }


 }
] )

This will return an array of the matching comments.

answered Dec 17, 2021 at 7:38

Rachit Arora

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