Why does populate return null in Mongoose?
The referenced ObjectId doesn't exist in the referenced collection, or the ref value in the schema doesn't match the model name. Check that the ref string matches the model name exactly and verify the referenced document exists.
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More FAQs in DevTinder Common Bugs and Solutions Debugging Guide for Node.js Projects
Check if MongoDB is running (mongosh for local), verify the MONGODB_URI in .env, ensure Atlas IP whitelist includes your IP, and check that the database name is in the connection string.
Ensure the frontend uses withCredentials: true in Axios, the backend sets credentials: true in CORS, and cookie options include httpOnly: true, sameSite: 'lax', and secure: true only in production (HTTPS). sameSite: 'none' requires secure: true.
Configure CORS on the backend Socket.io server with the frontend origin and credentials: true. On the frontend, use withCredentials: true and pass the token in auth. In production, configure Nginx with Upgrade and Connection headers for WebSocket support.
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