"NSInternalInconsistencyException" Entities for a configuration must already be in the model

Question

I am trying to add a new entity in NSManagedObjectModel in my NSIncrementalStore Subclass. I am doing this in loadMetadata method but it keeps throwing this exception on the last line. See Code Below

"NSInternalInconsistencyException" Entities for a configuration must already be in the model

Code

var model:AnyObject=(self.persistentStoreCoordinator?.managedObjectModel.copy())!
var newEntity=NSEntityDescription()
newEntity.name="newEntity"
newEntity.managedObjectClassName="newEntity"

var entities=model.entitiesForConfiguration(self.configurationName)
entities?.append(newEntity)

model.setEntities(entities!, forConfiguration: self.configurationName)

Answer 1

You cannot modify a model after it has been added to a persistent store coordinator. The only time you can manipulate the model is just after initialization and before applying it to the NSPersistentStoreCoordinator.

Answer 2

The documentation is unclear about this, but before calling setEntities:forConfiguration: the entities being set must already exist in the managed object model's entities array. This is because this method actually assigns existing entities in the model to a particular configuration.

The solution here is to make a mutable copy of the entities array, add your entities to it if they do not exist, and then set the managed object model's entities array to an immutable copy of the modified array. After that point you can invoke setEntities:forConfiguration:.

It would be worth filing a radar bug report on this behavior.