http://cyberfox.com/blog/120-how-i-learned-to-stop-worrying-about-core-data-and-love-macruby

entity.m

//
//  ObjCEntity.m
//  Entity
//
//  Created by dbrown on 2/10/11.
//  Copyright 2011 __MyCompanyName__. All rights reserved.
//

#import "ObjCEntity.h"

@interface ObjCEntity (private)

+ (NSArray*)findFromContext:(NSManagedObjectContext*)moc 
                 withEntity:(NSString*)ename 
              withPredicate:(NSPredicate*)pred 
                    options:(NSDictionary*)opts;

+ (NSPredicate*)extractPredicate:(NSDictionary*)opts;

@end

@implementation ObjCEntity

NSString* entityName = @"";

+ (void)initialize {
    entityName = NAMEOF([self class]);
}

+ (NSArray*)findFirstWithContext:(NSManagedObjectContext*)moc 
                         options:(NSDictionary*)opts {
    NSPredicate* pred = [self extractPredicate:opts];
    NSMutableDictionary* newOpts = [opts mutableCopy];
    [newOpts setValue:[NSNumber numberWithInt:1] forKey:@"limit"];
    return [self findFromContext:moc 
                      withEntity:entityName 
                   withPredicate:pred 
                         options:newOpts];
}

+ (NSArray*)findAllWithContext:(NSManagedObjectContext*)moc 
                       options:(NSDictionary*)opts {
    NSPredicate* pred = [self extractPredicate:opts];
    return [self findFromContext:moc 
                      withEntity:entityName 
                   withPredicate:pred 
                         options:opts];
}

+ (NSArray*)findFromContext:(NSManagedObjectContext*)moc 
                 withEntity:(NSString*)ename 
              withPredicate:(NSPredicate*)pred 
                    options:(NSDictionary*)opts {
    NSFetchRequest* fetchRequest = [[[NSFetchRequest alloc] init] autorelease];
    NSEntityDescription* entityDescription = [NSEntityDescription entityForName:ename
                                                         inManagedObjectContext:moc];
    
    [fetchRequest setEntity:entityDescription];
    [fetchRequest setPredicate:pred];
    
    NSNumber* offset = [opts objectForKey:@"offset"];
    NSNumber* limit = [opts objectForKey:@"limit"];
    
    if (offset) {
        [fetchRequest setFetchOffset:[offset intValue]];
    }
    
    if (limit) {
        [fetchRequest setFetchLimit:[limit intValue]];
    }
    
    NSError* error;
    return [moc executeFetchRequest:fetchRequest error:&error];
}

+ (NSPredicate*)extractPredicate:(NSDictionary*)opts {
    NSMutableArray* preds = [NSMutableArray array];
    NSMutableArray* pred_opt = [NSMutableArray array];
    NSMutableArray* byLines = [NSMutableArray array];
    for (NSString* key in [opts allKeys]) {
        if ([key hasPrefix:@"by_"]) {
            NSString* searchKey = [key substringFromIndex:3];
            [preds addObject:[NSString stringWithFormat:@"%@ = %%@", searchKey]];
            [pred_opt addObject:[opts valueForKey:key]];
            [byLines addObject:key];
        } else if ([key isEqualToString:@"conditions"]) {
            NSArray* cond = [opts objectForKey:key];
            [preds addObject:[cond objectAtIndex:0]];
            [pred_opt addObject:[cond subarrayWithRange:NSMakeRange(1, [cond count] - 1)]];
            [byLines addObject:key];
        }
    }
    
    if ([preds count] == 0) {
        return nil;
    }
    
    NSString* predString = [preds componentsJoinedByString:@" AND "];
    NSPredicate* result = [NSPredicate predicateWithFormat:predString argumentArray:pred_opt];
    
    [opts removeObjectsInArray:byLines];
    
    return result;
}

@end

I admit to being unsure how to translate from the original, so I ended up with the subArrayWithRange: stuff. Yes, I think your version would do the trick.

I think we've got a basic difference in preference here. I come from a Lisp and Smalltalk background -- the way that the arguments are distributed through the method name allows me to make complete sentences, rather than mapping everything into a prefix form. I can appreciate what you are saying, I know that I just tend to prefer my own code. Go figure. ;)

Thank you for the compliments. But remember to TEST THE CODE. I have not run this, it was a straightforward translation of your ruby code. I wanted to back up my "I don't see any reason this needed to be in Ruby" comment with an example.