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by Keith Ray.
Original Post: Using Cocoa's Reflection APIs for Reading/Writing an SQL Database
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CoreData is probably doing something like this already. If you don't like CoreData's use of sqlite3/XML or being limited to the Tiger version of MacOS X, you can probably use these APIs to implement your own object-to-database mapping. If you want to work with a database schema that doesn't map simply to your classes, look elsewhere. Also: I'm just getting to know SQL-based databases, so I may be mis-using SQL/DB terminology or concepts.
Writing to the database:
1. Call the method "className" to get the object's class-name and map that to a database table. You probably also want to map object addresses to a unique id in that database table.
2. Calls these methods to find keys of the object and map those to column-names (and possibly foreign keys to other tables): "attributeKeys", "toOneRelationshipKeys", "toManyRelationshipKeys"
3. Get values for keys by calling "valueForKey:" or "dictionaryWithValuesForKeys:". There's also "mutableArrayValueForKey:" for dealing with to-many relationships.
4. Assemble SQL from the classname/tablename, key-names/column-names, and values gotten above, and use that to write to the database. You'll probably need to write multiple rows to multiple tables to persist a complex aggregate object, thus you need to use techniques common to object-serialization to detect cycles and avoid writing an object to the db more than once. One of the PLoP books had a good discussion of serialization.
Reading from the database is pretty much the reverse of the above, but getting to the class from a classname/tablename requires calling NSBundle's "classNamed:" method or some other method or function I haven't found in my quick search using AppKiDo while writing this. Once you have the values and key-names, set them with the method "setValue:forKey:" or "setValuesForKeysWithDictionary:".
Cocoa has bunches of interesting methods like "classForArchiver" and "replacementObjectForArchiver:" that are probably used behind the scenes for implementations of Distributed Objects, reading/writing NIB files (in various NIB file formats, which recently included XML, I believe), and more recently, Core Data. Maybe someday I'll get more familiar with these. Oh, I haven't actually done this (yet), so don't ask me for help if you get stuck. :-)
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