Developer shares step-by-step guide to migrating Spring Boot app from MySQL to MongoDB

A developer documented every code change required to migrate a Task Manager project from MySQL to MongoDB in Spring Boot, after spending an evening debugging a mixed-dependency error. The migration involved replacing two Maven dependencies — the JPA starter and MySQL connector — with a single MongoDB starter, while fully removing the old JPA starter to avoid silent runtime conflicts. Application properties shrank from five database configuration lines to a single MongoDB URI, eliminating schema and dialect settings since MongoDB creates collections automatically. At the model layer, JPA annotations like @Entity, @Table, @GeneratedValue, and @ManyToOne were replaced with MongoDB equivalents such as @Document, @DBRef, and Spring Data's own @Id, with the id field type changing from Long to String. The author highlighted a subtle but critical pitfall: the @Id annotation exists in both JPA and Spring Data packages, and importing the wrong one causes MongoDB to silently ignore the Java field as the document identifier, breaking findById queries.