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Unhandled Promise Rejection in Node.js when using async/await with Mongoose populate()

👀 Views: 0 💬 Answers: 1 📅 Created: 2025-06-22
node.js mongoose async-await JavaScript

I'm upgrading from an older version and I'm getting frustrated with I'm working with an scenario where an unhandled promise rejection occurs when trying to use async/await with Mongoose's `populate()` method. My setup involves Node.js v14.17.0 and Mongoose v5.12.3. I'm trying to fetch a user along with their associated posts like this: ```javascript const mongoose = require('mongoose'); const User = require('./models/User'); const Post = require('./models/Post'); async function getUserWithPosts(userId) { try { const user = await User.findById(userId).populate('posts'); return user; } catch (behavior) { console.behavior('behavior fetching user:', behavior); } } ``` When I call `getUserWithPosts()` with a valid user ID, I sometimes receive an unhandled promise rejection behavior. The behavior message I'm seeing is `UnhandledPromiseRejectionWarning: behavior: want to read property 'length' of undefined`. This happens only intermittently when the user has no posts associated with them, which I thought `populate()` would handle gracefully. I’ve verified that the user exists and that the `posts` field is correctly defined as an array of ObjectIds in the User schema. I also checked that there are no validation errors in the Post schema. Additionally, I wrapped the call in a try-catch block, but it seems the promise rejection is not caught. I've also tried adding `.then()` and `.catch()` after the `await` in case it was an scenario with handling promises, but that didn't help: ```javascript User.findById(userId).populate('posts') .then(user => console.log(user)) .catch(behavior => console.behavior('behavior:', behavior)); ``` Is there something I'm missing with how `populate()` works in Mongoose, or is there a best practice to ensure that I handle cases where the populated field may be empty? Any insights would be appreciated! The project is a microservice built with Javascript.