What are common API relationship mistakes in MongoDB?
N+1 queries, populating without select, deep nested populate, storing refs as strings, no index on ref fields, embedding unbounded arrays, not handling null populate, denormalizing without a staleness plan, no cascade delete, and returning internal fields.
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More FAQs in Common API Relationship Mistakes in MongoDB (and How to Fix Them)
Without select, populate returns the entire referenced document, including sensitive fields like passwordHash. Use .populate('authorId', 'firstName lastName photoUrl') to return only what the client needs.
If you populate a ref that was deleted, the field is null. Check for null before accessing the populated field. For critical relationships, consider cascade delete or a soft delete with a deletedAt flag.
populate breaks. Mongoose expects ObjectId for ref fields. Store as mongoose.Schema.Types.ObjectId with ref. Mongoose casts strings to ObjectId in queries, but storing as string defeats the type safety and indexing benefits.
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