Entity Relationships

"Things"

Relationships

(E:entity, Em: Embeddable)

@ElementCollection :

• @Embeddable

  • cannot be queried,

  • does not have ID,

  • no version,

  • no reverse pointer...

http://en.wikibooks.org/wiki/Java_Persistence/ElementCollection

Many-to-One

@ManyToOne

Many-to-Many

@ManyToMany

Direction

Bi-directional relationship:

• each entity has a relationship field or property that refers to the other entity

• owning side

  • determines how the runtime makes updates to the relationship in the database

  • is

    • the "many" side of "many-to-one" bi-directional is always the owning side

    • in "one-to-one", the owning side contains the foreign key property

    • in "many-to-many", either side can be the owning side

• inverse side

  • must use mappedBy element of the relationship annotation (such as @OneToMany) to designate the property or field in the owning entity

  • is never the "many" side of "many-to-one"; "many" side is always the owning side

Uni-directional: only has owning side

Query

direction in relationship also determine if query can navigate from one entity to another

Cascade operation

(when one entity.... related entity also...):

• ALL = all

• DETACH : detach from persistence context

• MERGE : merge into persistence context

• PERSIST : persist into persistence context

• REFRESH : refresh in persistence context

• REMOVE : remove from persistence context

Use in @OneToOne or @OneToMany

Use orphanRemoval = "true" in @OneToOne or @OneToMany to specify removal of a child if the child is removed from the collection