Core Data Migration: Attribute Mapping Value Expression

Question

I currently have a cardType attribute on my entity, which in the old model could be \"Math\", \"Image\" or \"Text\". In the new model, I\'ll be using just \"Math\" and \"Text\"

Answer 1

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