Why use .lean() and .select() in a feed API?
.lean() skips hydrating full Mongoose documents (faster, less memory). .select() limits fields to what the client needs (smaller payload). Together they make the feed API faster and lighter.
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More FAQs in How to Build a Feed API in Node.js
GET /feed?skip=&limit=. Exclude the current user and already-swiped users. Cap the limit. Use .lean() and .select(). Return total. Run find and countDocuments in parallel with Promise.all. Index the filter fields.
Fetch the swiped user IDs with ConnectionRequest.find({ fromUserId }).distinct('toUserId'). Then filter the feed with _id: { $nin: [req.user.userId, ...swipedIds] }. Excludes the current user and anyone already swiped on.
To prevent abuse. Without a cap, a client can request 10000 items, eating memory and bandwidth. Use Math.min(parseInt(req.query.limit) || 20, 100). Most UIs only show 20 at a time anyway.
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