jmcnally 2002/06/22 20:57:42 Modified: xdocs user-guide.xml Log: patch by Jeff Linwood <[EMAIL PROTECTED]> Gives more detail on general OR mapping strategies. Revision Changes Path 1.7 +50 -8 jakarta-turbine-torque/xdocs/user-guide.xml Index: user-guide.xml =================================================================== RCS file: /home/cvs/jakarta-turbine-torque/xdocs/user-guide.xml,v retrieving revision 1.6 retrieving revision 1.7 diff -u -r1.6 -r1.7 --- user-guide.xml 8 Nov 2001 15:56:52 -0000 1.6 +++ user-guide.xml 23 Jun 2002 03:57:42 -0000 1.7 @@ -386,18 +386,60 @@ </section> -<section name="The Object Relational Map"> +<section name="Inheritance and the Object Relational Map"> <p> - The OM/Peer (Torque) system can map a class hierarchy into a single table. - This is 1 of 3 methods generally described in object-relational mapping - designs. Its benefits include that it does not require joins in order to - gather the attributes for a single object. It falls short in modelling a - class hierarchy where the related classes have a non intersecting collection - of attributes, as in this case a row in the table will have several null - columns. + Torque can handle object-oriented inheritance. There are generally + considered to be 3 methods of object-relational mapping designs. Torque + uses one of the fastest, mapping all objects in a class hierarchy to a + single table. All attributes for every class in the hierarchy are stored + in the table. Consider an abstract ComputerComponent class that has Monitor + and Keyboard subclasses. There would only be one table - both Monitor and + Keyboard objects would be persisted to the same place. The table would + consist of all ComputerComponent attributes, any unique Monitor attributes, + and any unique Keyboard attributes. Keyboard table rows would have NULL for + any unique Monitor data columns, and vice versa. + +</p> +<p> + + The other fast method is to map each concrete class to a distinct + table. Every object stores all attributes in a single row in the class table. + An example would be that if we had a Kitchen class that inherited from Room, + two tables would be needed for storage. The Kitchen table would contain all + of the columns of the Room table, plus any additional data columns needed to + describe the additional Kitchen attributes. + +</p> +<p> + + The slowest, but most object-oriented method is to store each class in its + own table. Only attributes that are added to a derived class are stored in + its table. The persistence layer would need to join tables to read an object + out of storage. Saving objects would be more complex, because objects will + need to be distributed across multiple tables. For our Kitchen and Room + example, there would also be two tables, Kitchen and Room, but the Kitchen + table would only contain those attributes which weren't part of the Room + class. + +</p> +<p> + + One of the advantages of the first method (the one Torque uses) is that it + does not require joins like the third method described above. Another + advantage is that the data model is easier to maintain than the second + method. It falls short in modelling a class hierarchy where the related + classes have a non intersecting collection of attributes, as in this case + a row in the table will have several null columns. + +</p> +<p> + For more information, visit Scott Ambler's excellent web site, + <a href="http://www.ambysoft.com/";>AmbySoft.com</A>, where he discusses + object mapping to relational databases. + </p> </section>
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