How to create Entity dynamically in Objective-C?

Question

I'm building an iPad application where I need user to create entity dynamically. I'm already having 3 entities which program uses.
Could you help me with code how to do it?

I want to understand the whole structure according to my understanding I have to create new managedObjectModel, add new entities and than merge it with existing one, is it correct?

Answer 1

That's correct -- you'd create an array of NSEntityDescription objects, then call setEntities: on the new managed object model. Then, finally, you'd merge that model with your built-in model.

But note that you can't change a model once it has been used to create a managed object context (or used for storage). You'll need to create new storage and context after the model is changed.

Answer 2

While it is possible to create a new entity and a new model on the fly in practice this is massively complex. If nothing else you would have to migrate any existing persisted data to the new model and a new persistent store file. I strongly recommend against attempting this especially if you are just starting out with Core Data.

You do have options:

Firstly, are you sure you actually need a new entity? People just starting out with Core Data often mistake entities for managed objects. Entities are to managed objects as classes are to instances. Entities are abstractions used to create the object graph. They don't actually contain data. The times when you need new entities are very,very rare.

Secondly, if you do need some kind of dynamic entity, it would usually be best to decompose the dynamic entity into numerous fixed subentities and then use relationships to create a virtual entity. Eg you need a dynamic Person "entity" so you create several entities in the model each of which holds one attribute of the person. You could have a Field entity which would have a fieldName attribute and then a fieldValue attribute. Then have a an actual Person entity that has no attributes but just relationships to the necessary Field objects. You could add any fields needed to any person and then reconstitute an virtual person object by walking the relationships to its fields.

I rather doubt however that you need that kind of flexibility. Such a need is very rare. I would step back and see exactly what dynamic data you think the user might need to enter.