Data Mapper Pattern

Question

Up until now I\'ve been using Active records in all my c# database driven applications. But now my application requires my persistence code being split from my business objects.

Answer 1

Shaun I would answer your questions this way:

ad 1) Mapper is responsible for creating Customer object. Your Mapper object will have something like RetrieveById method (for example). It will accept an ID and somehow (that't he responsibility of the Mapper object) construct the valid Customer object. The same is true the other way. When you call Mapper.Update method with a valid Customer object, the Mapper object is responsible for making sure that all the relevant data are persisted (wherever appropriate - db, memory, file, etc.)

ad 2) As I noted above retrieve/persist are methods on Mapper object. It is its responsibility to provide such a functionality. Therefore LoadAll, SaveAll (probably passing an array of value objects) are valid Mapper methods.

ad 3) I would say yes. But you can separate various aspects of Mapper objects into separate classes (if you want to/need to): default values, rule validation, etc.

I hope it helps. I really suggest/recommend you to read Martin Fowler's book Patterns of Enterprise Application Architecture.