How to Define Relationships in Hibernate 4 using Annotations?

Annotating Relationships in Hibernate 4

In Hibernate 4 and Spring, you can define various relationships between classes using annotations. The following outlines how to create one-to-one, one-to-many, many-to-one, and many-to-many relationships:

Uni-directional One-to-One Relationships:

• Use @OneToOne on the owning class (primary key of the owning class references the other class).
• No corresponding mapping required on the other class.

Bi-directional One-to-One Relationships:

• Use @OneToOne(mappedBy = "bar") on the owning class to indicate the relationship is managed by the other class.
• Use @OneToOne on the other class with mappedBy pointing to the owning class.

Uni-directional One-to-Many Relationships Using User Managed Join Table:

• Use @OneToMany on the owning class.
• Use @JoinTable to define the join table between the entities.
• No mapping required on the other class.

Bi-directional One-to-Many Relationship Using Foreign Key Mapping:

• Use @OneToMany(mappedBy = "bar") on the owning class.
• Use @ManyToOne on the other class with mappedBy pointing to the owning class.

Bi-directional Many-to-Many Using Hibernate Managed Join Table:

• Use @OneToMany on both classes to create a self-referencing relationship.
• Use @JoinTable to define the join table.

Bi-directional Many-to-Many Using User Managed Join Table Object:

• Use @OneToMany on both classes to create a self-referencing relationship.
• Define a separate class as the join table object.

Determining the "Owning" Side of Bi-directional Relationships:

• Typically, the class that has a collection of another class is considered the "owning" side.
• This determines where the foreign key is stored and which class manages the changes.

Selecting FetchType

• Lazy fetching (default) loads the related objects only when needed.
• Eager fetching (set fetchType = FetchType.EAGER) loads the related objects immediately.

Avoid LazyInitializationException by Using Hibernate.initialize() or FetchMode:

• Use Hibernate.initialize(person.getRoles()); to eagerly load a lazily fetched collection.
• Use setFetchMode("roles", FetchMode.SUBSELECT) on the criteria to eager load a collection.

Determining Cascade Direction:

• Use cascade to specify which operations (create, update, delete) cascade between the related entities.
• Set cascade both ways if required, but only if it makes sense semantically.

Orphan Removal:

• Enable orphanRemoval on @OneToMany to automatically delete orphaned entities (entities that are no longer referenced).

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