Core Data Migration: Attribute Mapping Value Expression

One thing you can do is create a custom migration policy class that has a function mapping your attribute from the original value to a new value. For example I had a case where I needed to map an entity called MyItems that had a direct relationship to a set of values entities called "Items" to instead store an itemID so I could split the model across multiple stores.

The old model looked like this:

The new model looks like this:

To do this, I wrote a mapping class with a function called itemIDForItemName and it was defined as such:

@interface Migration_Policy_v1tov2 : NSEntityMigrationPolicy {  NSMutableDictionary *namesToIDs;}- (NSNumber *) itemIDForItemName:(NSString *)name;@end

#import "Migration_Policy_v1tov2.h"@implementation Migration_Policy_v1tov2    - (BOOL)beginEntityMapping:(NSEntityMapping *)mapping manager:(NSMigrationManager *)manager error:(NSError **)error {                namesToIDs = [NSMutableDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithInt:1],@"Apples",                      [NSNumber numberWithInt:2],@"Bananas",                      [NSNumber numberWithInt:3],@"Peaches",                      [NSNumber numberWithInt:4],@"Pears",                      [NSNumber numberWithInt:5],@"Beef",                      [NSNumber numberWithInt:6],@"Chicken",                      [NSNumber numberWithInt:7],@"Fish",                      [NSNumber numberWithInt:8],@"Asparagus",                      [NSNumber numberWithInt:9],@"Potato",                      [NSNumber numberWithInt:10],@"Carrot",nil];        return YES;    }    - (NSNumber *) itemIDForItemName:(NSString *)name {                NSNumber *iD = [namesToIDs objectForKey:name];                NSAssert(iD != nil,@"Error finding ID for item name:%@",name);                return iD;    }    @end

Then for the related Mapping Name for the attribute in your mapping model you specify the Value Expression as the result of your function call as such:FUNCTION($entityPolicy,"itemIDForItemName:",$source.name) **

You also have to set the Custom Policy Field of your Mapping Name for that attribute to your mapping class name (in this case Migration_Policy_v1tov2).

**note this should match the selector signature of the method