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Persistent Objects In the last couple of articles we have introduced business objects and prada handbag shown how simple relationships can be represented. The relationships expressed as object properties are a more formal (and neutral) representation of the relationships between data entities that can be implicit within the database design, or explicit within the database schema. Moving away from a datacentric view of entities and relationships to a more structured, objectoriented exposition has many benefits, but most languages prada handbags outlet (including Delphi) cater only for an inmemory object model; objects have no state persistence capability unless one is provided for them.
There are many design solutions to this requirement, and we have already established that the business (problem domain) objects themselves should not communicate directly with the database. An appropriate design is to have another set of classes that exist purely as an object interface to the database, and will be responsible for loading and saving problem domain object state. Our problem ysl bags domain objects will know nothing about how their state is stored on disk, cheap ysl handbags only that there is a particular class that is responsible for this operation. The business objects will make a method call onto such an object and delegate the work to it.
The first decision that must be taken is to decide how to map objects onto a database schema. Although a number of object databases exist, most companies have standardised on an existing RDBMS isabel marant shoes with which they are comfortable and experienced. Promoting a development change to a more objectoriented balenciaga bag approach is difficult enough without demanding that the typically large investment in a highly evolved database technology is replicated for a new, unfamiliar one. Therefore, this article will focus on mapping objects to a SQLbased RDBMS, as this constitutes the majority of installed development systems. It should, however, be stressed that one of the advantages of our object model is that it is totally architectureneutral and can be applied to a large number of different database topologies, including ISAM and objectbased ones.
It is obvious and natural that there is a relatively simple conceptual correspondence between an object and a database tuple (record). Therefore, when storing object state in a RDBMS a very common, and practical, solution is to map a problem domain classes to tables, objects to records and properties to fields. Note that because our balenciaga bags on sale database will isabel marant shoes for sale be storing object state, it must be a complete representation and therefore it is likely that there will be more fields in the database table than a given class has public properties. In some cases a public property may not have a directly corresponding field, but the general principle is to map a property onto a database field of similar fundamental type (character, numeric, date etc.)