We used RestKit to build 55Prophets (available in the app store). Restkit's readme page has some instructions for mapping error responses, but I ran into several problems along the way, so I came up with my own setup for taking errors that come out of our back end and presenting them to a user.

To start, I created a class called ErrorCollection. It is relatively simple and the header file looks like this:

// ErrorCollection.h
#import
@interface ErrorCollection : NSObject

@property (nonatomic, strong) NSArray *messages;
@property (nonatomic, strong) NSString *message;

-(NSString *)messagesString;

@end

The implementation is also simple. It looks like this:

// ErrorCollection.m
#import "ErrorCollection.h"

@implementation ErrorCollection

  -(NSString *)messagesString{
    if (self.message) {
      return self.message;
    }
    else{
      NSMutableString *str = [[NSMutableString alloc] init];
      for (int i=0; i<self.messages.count; i++) {   
        NSString *msg = [self.messages objectAtIndex:i];
        if (i==self.messages.count-1) {
          [str appendFormat:@"%@", msg];
        }
        else {
          [str appendFormat:@"%@\\n", msg];
        }
      }
        
      return str;
    }
  }

@end

Our back-end rails app either returns json in the form of {"errors": ["error1", "error2" ... ] } or {"error": "error message" }. In the case of the former, we want to map each error string in the array to a string in the messages array of an ErrorCollection. In the case of the latter, we want to map the string to the message property of the ErrorCollection. Then, we can use messagesString method, which will give us a single string to present to the user, containing the error message or messages.

To get RestKit to use the ErrorCollection class, we need to add a response descriptor for it. Wherever you set up your response descriptors, add something like this:

RKObjectMapping *errorMapping = [RKObjectMapping mappingForClass:[ErrorCollection class]];

[errorMapping addPropertyMapping:[RKAttributeMapping attributeMappingFromKeyPath:@"errors" toKeyPath:@"messages"]];

[errorMapping addPropertyMapping:[RKAttributeMapping attributeMappingFromKeyPath:@"error" toKeyPath:@"message"]];

[[RKObjectManager sharedManager] addResponseDescriptor:[RKResponseDescriptor   responseDescriptorWithMapping:errorMapping pathPattern:nil keyPath:nil   statusCodes:RKStatusCodeIndexSetForClass(RKStatusCodeClassClientError)]];

Now you can access the error collection in the failure block many of the methods of the RestKit object manager. For example:

[[RKObjectManager sharedManager] postObject:someObject path:@"/object_path" parameters:nil
  success:^(RKObjectRequestOperation *operation, RKMappingResult *mappingResult){
  // handle success
}
failure:^(RKObjectRequestOperation *operation, NSError *error){
  ErrorCollection *errors = [[[error userInfo] objectForKey:RKObjectMapperErrorObjectsKey] lastObject];
  NSLog(@"%@", [errors messagesString]);
}];