How to limit the number of documents in a MongoDB request in Node.js ?

Last Updated : 20 Oct, 2021

Say you are working on a quiz API, with which anybody on the web can fetch random questions based on the defined category. You are done with your API and it has passed all test parameters, now for sake of users you want to add a feature that will let users decide how many questions they want to address and your API will provide them with that amount of questions. Here, comes the helpful mongo feature limit.

Referring to mongo documentation, the limit can be understood as an option that limits the number of documents that has to be passed to the next pipeline for processing.

Syntax:

{ $limit: <positive integer> }

For Node.js we will use mongoose for this specific purpose. Assuming that you created a Post model for a Content Management System, let’s try fetching a certain number of posts using the find() method.

Javascript

Post.find().limit(5).then(posts=>{ 
    if(!posts){ 
        const error=new Error("No post is there to fetch"); 
        error.statusCode=404; 
        throw(error); 
    } 
    return res.status(200).json({post:posts}); 
}).catch(err=>console.log(err));

Pass the number of documents you want to fetch as an argument to limit. That’s a quite abstraction right? Worry not let’s walk through the process from the very beginning.

Assuming a decent foundation of Node JS, express and mongoose. Starting off with creating posts and fetching posts as per defined limit.

Workflow for follow along will be:

Setting up environment
Creating Post schema with mongoose
Developing API for post creation
Developing API for fetching the specified number of post from database

Setting up:

Boiler Plate:

touch app.js

Setting up the NPM setup:

npm init

Installing dependencies:

npm i express body-parser mongoose

Project structure: Now, your file tree will look somewhat.

Follow the below steps one by one to achieve the target:

Step 1: Setting up the app.js.

Filename: app.js

Javascript

const express = require("express");  
const bodyParser = require("body-parser");  
const mongoose = require("mongoose");  
    
const app = express();  
    
// For processing application/json  
app.use(bodyParser.json());  
    
// Setting up CORS  
app.use((req, res, next) => {  
  res.setHeader("Access-Control-Allow-Origin", "*");  
  res.setHeader("Access-Control-Allow-Methods", "POST,PUT");  
  res.setHeader("Access-Control-Allow-Headers", "*");  
  next();  
});  
    
app.get("/test", (req, res, next) => {  
  res.send("<h1>working</h1>");  
});  
    
mongoose  
  .connect("mongodb://localhost:27017/articleDB")  
  .then(app.listen(8000, () => console.log("listening at port 8000")));

Run the app.js file

node .\app.js

Output: Now on hitting localhost:8000/test, you should expect.

Now, when we are done with our basic app setting, lets create our required APIs.

Creating Post Schema: Schemas can be easily created in Node using mongoose.

Javascript

// Requiring mongoose 
const mongoose = require("mongoose"); 
  
// Schema of our post model 
const postSchema = new mongoose.Schema( 
  { 
    title: { 
      type: String, 
      required: true, 
    }, 
    content: { 
      type: String, 
      required: true, 
    }, 
  }, 
  { timestamps: true } 
); 
  
// Model 
const Post = mongoose.model("Post", postSchema);

API for post creation: Now, as we are done it creating the model and schemas we will move forward creating our first API and then testing it using Postman.

Javascript

// POST http://localhost:8000/post 
app.post("/post",(req,res,next)=>{ 
    const newPost = new Post({ 
    title: req.body.title, 
    content: req.body.content, 
  }); 
  newPost 
    .save() 
    .then((post) => { 
      return res.status(201).json({ post: post }); 
    }) 
    .catch((err) => { 
      res.status(500).json({error:err}); 
    }); 
})

URL - http://localhost:8000/post
Method - POST
Content-Type - application/json
Body of the request should contain -
i. title - title of your article
ii. content - content of your article

Output:

API for fetching post as per the queried number of posts: Idea is to pass the count as a query in the API endpoint and then processing that query using basic validation and limit.

Javascript

// GET http://localhost:8000/posts 
  
app.get("/posts",(req,res,next)=>{ 
  const requestCount = req.query.count; 
  Post.find() 
    .countDocuments() 
    .then((count) => { 
      // if requested count is more than actual count of posts in database 
      if (requestCount > count) { 
        const error = new Error("invalid request in quantity"); 
        error.statusCode = 400; 
        throw error; 
      } 
      //returning posts while limiting the count 
      return Post.find().limit(Number(requestCount)); 
    }) 
    .then((posts) => { 
          res.status(200).json({ posts: posts }); 
    }) 
    .catch((err) => { 
        const status=error.statusCode || 500; 
        res.status(status).json({error:err}); 
    }); 
})

limits only accepts a number as an argument but by default query parameters is of string type which required us to typecast it into number.

Post.find().countDocuments() returns total number of document present under that specific collection.

URL - http://localhost:8000/posts/?count={limit}
Method - GET
Example Request - http://localhost:8000/posts/?count=4

Output:

Final Solution:

Filename: app.js

Javascript

const express = require("express");  
const bodyParser = require("body-parser");  
    
// Requiring mongoose for interacting with mongoDB database  
const mongoose = require("mongoose");  
    
const app = express();  
    
// For processing application/json  
app.use(bodyParser.json());  
    
// Setting up CORS  
app.use((req, res, next) => {  
  res.setHeader("Access-Control-Allow-Origin", "*");  
  res.setHeader("Access-Control-Allow-Methods", "POST,PUT");  
  res.setHeader("Access-Control-Allow-Headers", "*");  
  next();  
});  
    
//---- mongo collection setup ----  
// schema of our post model  
const postSchema = new mongoose.Schema(  
  {  
    title: {  
      type: String,  
      required: true,  
    },  
    content: {  
      type: String,  
      required: true,  
    },  
  },  
  { timestamps: true }  
);  
    
// model  
const Post = mongoose.model("Post", postSchema);  
    
//for testing purpose - can ignore it  
app.get("/test", (req, res, next) => {  
  res.send("<h1>working</h1>");  
});  
    
//-----apis----------  
// POST http://localhost:8000/post  
app.post("/post", (req, res, next) => {  
  const newPost = new Post({  
    title: req.body.title,  
    content: req.body.content,  
  });  
  newPost  
    .save()  
    .then((post) => {  
      return res.status(201).json({ post: post });  
    })  
    .catch((err) => {  
      res.status(500).json({ error: err });  
    });  
});  
    
// GET http://localhost:8000/posts  
app.get("/posts", (req, res, next) => {  
  const requestCount = req.query.count;  
  Post.find()  
    .countDocuments()  
    .then((count) => {  
      // if requested count is more than actual   
      // count of posts in database  
      if (requestCount > count) {  
        const error = new Error("invalid request in quantity");  
        error.statusCode = 400;  
        throw error;  
      }  
      // returning posts while limiting the count  
      return Post.find().limit(Number(requestCount));  
    })  
    .then((posts) => {  
      res.status(200).json({ posts: posts });  
    })  
    .catch((err) => {  
      const status = err.statusCode || 500;  
      res.status(status).json({ error: err });  
    });  
});  
    
// connecting to database and serving up server  
mongoose  
  .connect("mongodb://localhost:27017/articleDB")  
  .then(app.listen(8000, () => console.log("listening at port 8000")));