Entity Mapping: Relationships

@ManyToOne on the child with @JoinColumn is the owning side that controls the FK; @OneToMany(mappedBy) on the parent is the inverse side for navigation only – always set both sides with helper methods.

The MANY side with @ManyToOne @JoinColumn owns the foreign key; the ONE side with @OneToMany(mappedBy=...) is the inverse, and mappedBy names the owning field on the child, not a column.

Order is the owning side (@ManyToOne @JoinColumn(user_id)); User is the inverse side (@OneToMany(mappedBy="user")); use cascade + orphanRemoval for a true aggregate and JOIN FETCH to avoid N+1.

Cascade propagates parent operations (PERSIST/MERGE/REMOVE/DETACH/REFRESH, or ALL) to children; orphanRemoval deletes a child when it leaves the parent's collection.

Only the owning side (the foreign key) is persisted; update both sides via helper methods so the in-memory object graph stays consistent with the database.