"An unwritten consensus in the IT industry is that data shared between object-oriented software and relational databases is best exchanged with object/relational (O/R) mapping frameworks where the entity relationship (ER) model follows the object-oriented model. This article proposes a reversed, lightweight approach supported by a small framework called Amber. This approach uses Java annotations to manage the CRUD cycle (Create Read Update Delete) of JavaBeans. Transaction management is put back into the database, and XML mapping descriptors are replaced by annotations. This article is for intermediate Java developers who are interested in efficient transactions with databases without XML descriptors."
"Playing with JDO 2 fetch groups, ZODB, thinking about TranQL (for two months now), playing with Prevayler, and looking at TORPEDO (need to run OJB against it when I have a chance) something clicked for me which I think clicked for some other people a long time ago -- but somehow got lost in the hullabaloo. We may all be doing O/R mapping wrong. Seriously, we probably are."
"Why a paper about mapping objects to relational databases? Because of the "impedance mismatch" between the object paradigm and the relational paradigm. The object paradigm is based on software engineering principles such as coupling, cohesion, and encapsulation, whereas the relational paradigm is based on mathematical principles, particularly those of set theory. The two different theoretical foundations lead to different strengths and weaknesses. Furthermore, the object paradigm is focused on building applications out of objects that have both data and behavior, whereas the relational paradigm is focused on storing data. The "impedance mismatch" comes into play when you look at the preferred approach to access: with the object paradigm you traverse objects via their relationships, whereas with the relational paradigm you duplicate data to join the rows in tables. This fundamental difference results in a less-than-ideal combination of the two paradigms, but then, a few hitches are to be expected. One of the secrets of success for mapping objects to relational databases is to understand both paradigms and their differences, and then make intelligent trade-offs based on that knowledge."
"Implementation of object-relational mapping (O/R mapping) is a general need for many projects in software development. Usually, work with the automation process of data storage is very boring, and at manual implementation there is a danger of mistakes and bugs. Also, to add constantly varying requirements, it is necessary for the developer to take into account the complex process of synchronization of all initial code and data storage structures. More than that, taking into account the necessity of portability of application between platforms, it all becomes even more complex and confusing."
"The mapping documents are compiled at application start-up to provide Hibernate with information about the persistent objects' corresponding classes, their respective structures, to which database table should they be mapped, and how. Hibernate also uses these mapping documents to generate corresponding database schema and stub Java classes for the persistence layer, using inbuilt utilities called SchemaExport and CodeGenerator, respectively."
