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

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 environmentCreating Post schema with mongooseDeveloping API for post creationDeveloping API for fetching the specified number of post from database
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 mongooseconst mongoose = require("mongoose");  // Schema of our post modelconst postSchema = new mongoose.Schema(  {    title: {      type: String,      required: true,    },    content: {      type: String,      required: true,    },  },  { timestamps: true });  // Modelconst 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/postapp.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")));
Output:Database, now we have our simple API with document limiting feature enabled.
surindertarika1234
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