re: Core Data: Cross-Model Relationships
re: Core Data: Cross-Model Relationships
- Subject: re: Core Data: Cross-Model Relationships
- From: Kai Brüning <email@hidden>
- Date: Thu, 15 Nov 2007 10:03:18 +0100
Ben,
thanks for your answer.
>Kai,
>
>>So far, so easy: +[NSManagedObjectModel modelByMergingModels:] will
>>merge my sub-models nicely. But I need some cross-sub-model
>>relationships. My idea is to create these relationships
>>programmatically after the sub-models have been loaded and merged
>>into one model.
>>
>>Questions:
>>
>>- Is this feasible and a good idea?
>>- Does my whole consideration make sense?
>>- Is there a better/easier way to achieve what I want?
>
>On the whole what you propose makes sense. There is one delicate consideration. When you open a store file, the model you use must match exactly the model used to create the store. No more, no less.
>
>So two points come up. Any programmatic alterations to a model must be maintained. If you add some code, and then save some files, you can't change your mind later about what programmatic alterations to make without performing a schema migration.
Yes, this was my understanding.
>
>Secondly, the sub-stores and sub-models cannot at runtime actively use a subset of the primary model. They either need to exactly use the primary model, or have their own NSPersistentStoreCoordinator with their own precise model.
Exactly. My idea is to load the sub-model(s) separately and create a whole separate Core Data stack to access export files.
>
>Or put another way, Core Data doesn't support dynamic plugins to the model without either a different coordinator, a schema migration step, or both.
That is an interesting and important way to phrase it. While technically more than understandable, this is a very unfortunate restriction of Core Data for modern application development.
Kai
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