CHAPTER 5 n AN OVERVIEW OF MAPPING n (Web design service)
CHAPTER 5 n AN OVERVIEW OF MAPPING n AN OVERVIEW OF MAPPING Primary Key Hibernate demands that a primary key be used to identify entities. The choice of a surrogate key, a key chosen from the business data, and/or a compound primary key can be made via the mapping file. When a surrogate key is used, Hibernate also permits the key-generation technique to be selected from a range of techniques that vary in portability and efficiency. The Use of SQL Formula Based Properties It is sometimes desirable that a property of an entity should be maintained not as data directly stored in the database, but rather as a function performed on that data for example, a subtotal field should not be managed directly by the Java logic, but instead maintained as an aggregate function of some other property. Mandatory and Unique Constraints As well as the implicit constraints of a primary or foreign key relationship, you can specify that a field must not be duplicated for example, a username field should often be unique. Fields can also be made mandatory for example, requiring a message entity to have both a subject and message text. The generated database schema will contain corresponding NOT NULLand UNIQUEconstraints so that it is literally impossible to corrupt the table with invalid data (rather, the application logic will throw an exception if any attempt to do so is made). Note that primary keys are implicitly both mandatory and unique. Cascading of Operations As alterations are made to the object model, operations on some objects should cascade through to related objects. For example, deleting a stocked item should perhaps cause any associated catalog entries to be deleted. The reverse deleting a single catalog entry should not necessarily cause the stocked item to be deleted from the database entirely! It would be awkward to manage the appropriate cascading rules from code alone, so cascading rules can be specified at a fine level of detail within the mappings. Summary This chapter has given you an overview of the reason why mapping files are needed, and what features they support beyond these absolute requirements. It has discussed the various types of associations, and the circumstances under which you would choose to use them. The next two chapters look at how mappings are specified using annotations and XML files respectively.
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