[Java] [JPA] @mapkey example

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key\value	@CollectionTable	uni-directional @OneToMany	bi-directional @OneToMany	@ManyToMany
@MapKeyColumn	link	link	link	link
@MapKeyTemporal	link	link	link	link
@MapKeyEnumerated	link	link	link	link
@MapKeyJoinColumn	link	link	link	link

• @MapKeyColumn if the map key is a basic type.
  If you don't specify the column name, the name of the property is «field»_KEY
• @MapKeyEnumerated/[@MapKeyTemporal](https://docs.oracle.com/javaee/7/api/javax/persistence/MapKeyTemporal.html @MapKeyTemporal) if the map key type is respectively an enum or a Date.
• @MapKeyJoinColumn/@MapKeyJoinColumns if the map key type is another entity.

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• If the map key is NON-basic entity, the map key MUST be persisted (saved) before persist the holder.
• If the relationship is many-to-many and the map value is NON-basic entity, the map value also MUST be persisted before persist the holder.

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• @ElementCollection is required if the map value is a basic type.
  Corresponding config annotation are @CollectionTable and @Column.
• if the map value is NON-basic type, it's required either @OneToMany or @ManyToMany.
  Corresponding config annotation is @JoinTable.