Foreign Keys: A must in SQL, but not in a Document Database?

Relational databases use foreign keys to enforce referential integrity and manage relationships between tables. Foreign keys ensure consistency and accuracy in join operations by linking related data. MongoDB, on the other hand, takes an application-centric approach to managing relationships. In MongoDB, related data with shared lifecycles can be embedded within a single document, eliminating the need for foreign keys for strong relationships. MongoDB's $lookup operation ensures consistency in join operations similar to a left outer join. When migrating from relational databases, `ON DELETE CASCADE` translates to embedding, and `ON DELETE SET NULL` to extended references. `ON DELETE RESTRICT` requires application logic for deletion management in MongoDB. Update actions should be avoided by utilizing an immutable `_id` for references. Many-to-many relationships are converted into arrays of references in MongoDB. MongoDB's data embedding and aggregation pipelines provide a flexible way to maintain document relationships and consistency.