Question

I am working on an iPad application, that requires me to store data locally if the user doesn’t have internet access and later on sync with the back-end database.

For local storage, I am planning to use Core Data with SQLite.

I am using Core Data for the first time and it seems, it retrieves entity and store entity in the form of a dictionary.

So should I be creating Model classes at all ?

What is a good design for such application.

I have a DataEngine class whose responsibility is to store entity on a server or local DB based on connectivity.

Now I am little confused If I need to create a Model class and ask individual model classes to save themselves using NSMangaedObjectContext with a dictionary representation Or just directly save data instead of creating a model object and asking it to do it ?

Should I be using a Moel class for each entity and that will server as interface between JSON representation that coms/goes from/to server.

and the representation I get from managedObjectContext.

Or shall I compeletely rely on the entity and relation ships that Core Data creates ?

Answer 1

I’ll do this backwards: first some things for you to check and then some ideas.

Check my own question.

I’d say that on your custom categories you can do the interface between the JSON representation and your classes.

You should also check RestKit which can do already much of what you need.

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You’re talking about two separate problems as far as I can understand:

1. Syncing local data to the server based on connectivity;
2. Using model classes.

Problem 1

I think you should have a class with the common code and each of your model classes should have its own mapping (to map between model and JSON) and saving methods.

Another class, that may be your DataEngine class, takes care of saving the right objects at the right time.
Take a look at RestKit as it helps with the mapping and the saving. I’m not sure about the syncing though.

Problem 2

I think you should have model classes. It helps a lot to work with objects and you have then a place to save methods for finding different kinds of data.

For this my question might be useful for you because you can create a CoreData model with generated class files and update it whenever you want while keeping your custom code.