How do I test APIs with relationships?
Use Jest and supertest. Test that populate returns the expected fields. Test that N+1 is avoided (check the number of queries in test logs). Test cascade deletes (deleting a user removes their posts). Test null populate (a deleted ref returns null, not a crash).
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More FAQs in Roadmap for Writing APIs With Relationships in Node.js
Understand refs, learn populate, learn the N+1 problem, learn nested populate, learn virtual populate, learn embed vs reference, learn denormalization, learn cascade deletes, learn side-loading, learn indexing for refs, learn pagination with populate, and test.
Refs in Mongoose. A ref field stores an ObjectId and references another model. Without refs, you cannot do populate or handle relationships. This is the foundation.
After populate and the N+1 problem. Denormalization is for high-traffic reads where populate is too slow. You copy key fields (author name on the post) and decide whether to update on change or accept staleness.
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