Fix to Marcus Zarra's consecutive migration code

AppDelegate.m.patch

--- 1.txt	2012-04-06 14:09:33.000000000 -0400
+++ 2.txt	2012-04-06 14:09:45.000000000 -0400
@@ -119,28 +119,35 @@
   }
   //END:progressivelyMigrateURLFindModels
 
   //See if we can find a matching destination model
   //START:progressivelyMigrateURLFindMap
   NSMappingModel *mappingModel = nil;
   NSManagedObjectModel *targetModel = nil;
   NSString *modelPath = nil;
   for (modelPath in modelPaths) {
     targetModel = [[NSManagedObjectModel alloc] 
-                   initWithContentsOfURL:[NSURL fileURLWithPath:modelPath]];
-    mappingModel = [NSMappingModel mappingModelFromBundles:nil 
-                                            forSourceModel:sourceModel 
-                                          destinationModel:targetModel];
-    //If we found a mapping model then proceed
-    if (mappingModel) break;
-    //Release the target model and keep looking
-    [targetModel release], targetModel = nil;
+                    initWithContentsOfURL:[NSURL fileURLWithPath:modelPath]];
+    // We gain nothing by migrating to the same model
+    if ([targetModel isEqual:sourceModel] ||
+      [[targetModel entityVersionHashesByName] isEqual:[sourceModel entityVersionHashesByName]]) {
+      // Ignore this model, it's the same one we're at and choosing it
+      // will lead to infinite recursion.
+    } else {
+      mappingModel = [NSMappingModel mappingModelFromBundles:nil 
+                                              forSourceModel:sourceModel 
+                                            destinationModel:targetModel]; 
+      //If we found a mapping model then proceed
+      if (mappingModel) break;
+      //Release the target model and keep looking
+      [targetModel release], targetModel = nil;
+    }
   }
   //We have tested every model, if nil here we failed
   if (!mappingModel) {
     NSMutableDictionary *dict = [NSMutableDictionary dictionary];
     [dict setValue:@"No models found in bundle" 
             forKey:NSLocalizedDescriptionKey];
     *error = [NSError errorWithDomain:@"Zarra" 
                                  code:8001 
                              userInfo:dict];
     return NO;