How does populate work in Mongoose?
You define a ref field that stores an ObjectId and references another model. On read, .populate('senderId', 'firstName') does an extra query to fetch the referenced document and replaces the ObjectId with the document. Always select specific fields.
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More FAQs in Mongoose ref and populate: A Deep Dive
Populating a field, then populating a field inside that. Example: .populate({ path: 'comments', populate: { path: 'authorId', select: 'firstName' } }). Each level does an extra query; use sparingly.
For relationships not stored as a field. userSchema.virtual('posts', { ref: 'Post', localField: '_id', foreignField: 'authorId' }). Then User.findById(id).populate('posts') looks up posts where authorId matches the user's _id.
Looping over documents and calling findById for each reference does N+1 queries. Use populate to do it in 2 queries (one for the parent, one for all referenced docs). Much faster.
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