I have an ugly situation where I need two datamodules (TDMA, TDMB) in a Delphi app. Each datamodule has the same data object names (queries, tables, etc.) but from a different component set (TZQuery, TADOQuery). I need this because I want to support multiple databases but not all databases are supported by my component suite. Which datamodule I need to access is determined by the DBFlag boolean variable. Besides having separate nearly identical code segments for each data access, is there some more efficient way?
If I could set a global datamodule variable like DMG to either DMA or DMB based on DBFlag then my code could reference DMG instead of DMA or DMB. That would be ideal and require very little code modifications but impossible as far as I know.
My suggestion is to drop building your DataModules based on specific datasets. Build them using only TClientDataSet and write all your code or link all your DataSources to these datasets. Then create other DataModules to hold your specific datasets and use your selection method to choose which one to respond as a data provider for the CDS instances. The idea of using an interface to do that is indeed a good one.
This approach will remove all you duplicate code and you will separate business logic (the code that handles the data inside the CDSs) from persistence (the code that transfers data rows from and to the data server).