add many to many bidirectional associations (@ManyToMany/mappedBy/@JoinTable)
add many to many unidirectional associations (@ManyToMany/@JoinTable)
add one to one unidirectional associations by foreign key (@OneToOne/@JoinColumn(unique = true))
add one to one bidirectional associations by primary key (@OneToOne/@PrimaryKeyJoinColumn)
add one to one unidirectional associations by primary key (@OneToOne/@PrimaryKeyJoinColumn)
add one to one bidirectional associations by foreign key (@OneToOne/mappedBy/@JoinColumn(unique = true))