This is an example on how to use MacRuby to read/write CoreData records. Tested with MacRuby 0.6 on Mac OS X 10.6.3.

Bash

$ /Developer/usr/bin/momc simpleTextMessageModel.xcdatamodel simpleTextMessageModel3.mom

cData-rear-write.rb

framework 'cocoa'
 
def fetchData (manObjCon, entity)
  fetch = NSFetchRequest.alloc.init
  fetch.setEntity(NSEntityDescription.entityForName(entity, inManagedObjectContext: manObjCon))
  fetch.setFetchLimit(25)
  manObjCon.executeFetchRequest(fetch, error: nil)
end

def storeData(manObjCon, entity, message, recipient1, recipient2, msgType)
  newMessage = NSEntityDescription.insertNewObjectForEntityForName(entity, inManagedObjectContext: manObjCon)

  newMessage.textMessage = message
  
  newRecipient = NSEntityDescription.insertNewObjectForEntityForName('toRecipient', inManagedObjectContext: manObjCon)
  newRecipient.personName = recipient1
  
  newRecipient2 = NSEntityDescription.insertNewObjectForEntityForName('toRecipient', inManagedObjectContext: manObjCon)
  newRecipient2.personName = recipient2  
  
  newType = NSEntityDescription.insertNewObjectForEntityForName('Type', inManagedObjectContext: manObjCon)
  newType.messageType = msgType
  newMessage.messageToType = newType

  set1 = NSSet.alloc.initWithObjects(newRecipient, newRecipient2, nil) #Storing two instances of recipient to show how to store records in a 1-many relationship
  newMessage.messageTo = set1
  manObjCon.save(nil) #Can pass an error object
end
 
def printData(manObjCon, entity)
  results = fetchData (manObjCon, entity)
 puts "Total records: #{results.length}" #Returns the number of messages available
 puts "Results class: #{results.class}" #Returns "Array"

  results.each do |result|
     puts "----------------------------"
    puts "Message: #{result.textMessage}"  #Returns the message text
    recipients = result.messageTo
    puts "recipients records: #{recipients.allObjects.count}" #Returns the number of recipients to this message
    puts "recipients type: #{recipients.allObjects.class}" #Returns "Array"
    
    msgType = result.messageToType.messageType
    puts "Message type: #{msgType}" #Returns the class of the message
    
    puts "Recipients array: #{recipients.allObjects.to_s}"
    recipients.allObjects.each do |recipient|
      puts "Recipient class: #{recipient.class}"  #Returns "NSManagedObject_toRecipient_"
      puts "Recipient: #{recipient.personName}"  #Return the recipient name
    end
    
   
  end
end

def createManagedObjectContext
  modelURL = NSURL.fileURLWithPath('simpleTextMessageModel3.mom')
  storeURL = NSURL.fileURLWithPath('codeDataTest3.sqlite')

  mom = NSManagedObjectModel.alloc.initWithContentsOfURL(modelURL)
  psc = NSPersistentStoreCoordinator.alloc.initWithManagedObjectModel(mom)
  ps = psc.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: storeURL, options: nil, error: nil)
  moc = NSManagedObjectContext.alloc.init
  moc.setPersistentStoreCoordinator(psc)
end

puts "Starting"

moc = createManagedObjectContext
puts "Store something"
storeData(moc, "message", "#{Time.now} - this is the message", "Dr Peter", "Dr John", "A simple message")
puts "Print the messages"
printData(moc, 'message')

message.h

#import <CoreData/CoreData.h>


@interface message :  NSManagedObject  
{
}

@property (nonatomic, retain) NSString * textMessage;
@property (nonatomic, retain) NSSet* messageTo;
@property (nonatomic, retain) NSManagedObject * messageToType;

@end


@interface message (CoreDataGeneratedAccessors)
- (void)addMessageToObject:(NSManagedObject *)value;
- (void)removeMessageToObject:(NSManagedObject *)value;
- (void)addMessageTo:(NSSet *)value;
- (void)removeMessageTo:(NSSet *)value;

@end

toRecipient.h

#import <CoreData/CoreData.h>

@class message;

@interface toRecipient :  NSManagedObject  
{
}

@property (nonatomic, retain) NSString * personName;
@property (nonatomic, retain) message * messageReceived;

@end

toRecipient.m

#import "toRecipient.h"

#import "message.h"

@implementation toRecipient 

@dynamic personName;
@dynamic messageReceived;

@end

Type.h

#import <CoreData/CoreData.h>

@class message;

@interface Type :  NSManagedObject  
{
}

@property (nonatomic, retain) NSString * messageType;
@property (nonatomic, retain) NSSet* typeToMessages;

@end


@interface Type (CoreDataGeneratedAccessors)
- (void)addTypeToMessagesObject:(message *)value;
- (void)removeTypeToMessagesObject:(message *)value;
- (void)addTypeToMessages:(NSSet *)value;
- (void)removeTypeToMessages:(NSSet *)value;

@end

Type.m

#import "Type.h"

#import "message.h"

@implementation Type 

@dynamic messageType;
@dynamic typeToMessages;

@end