Creating Mappings with Hibernate XML Files - Beginning Hibernate: From Novice to Professional - page 167

Mapping Other Associations
In Figure 7-3, the Advert class includes an instance of a Picture class. The relationship in the
tables is represented with the Picture table having a foreign key onto the Advert table.

Figure 7-3. Mapping an aggregation or composition relationship

A one-to-one correspondence does not absolutely require you to incorporate both parties
into the same table. There are often good reasons not to. For instance, in the Picture example,
it is entirely possible that while the initial implementation will permit only one Picture per
Advert , a future implementation will relax this relationship. Consider this scenario from the
perspective of the database for a moment (see Table 7-17).

Table 7-17. The Advert Table

ID Title Contents PictureCaption PictureFilename

advert001.jpg 1 Bike Bicycle for sale My bike (you can ride it if you like)

advert002.jpg 2 Sofa Sofa, comfy but used Chesterfield sofa

advert003.jpg

3 Car Shabby MGF for sale MGF VVC (BRG)

If we want to allow the advert for the sofa to include another picture, we would have to
duplicate some of the data, or include null columns. It would probably be preferable to set up
a pair of tables: one to represent the adverts, and one to represent the distinct tables (as
shown in Tables 7-18 and 7-19).

Table 7-18. The Refined Advert Table

ID Title Contents

1 Bike Bicycle for sale

2 Sofa Sofa, comfy but used

3 Car Shabby MGF for sale