Adding aggregate operation support submitted from Duane Fields. Thank…

…s for submitting!

README.md

@@ -79,6 +79,9 @@ Or, to have the results sorted by a property:
 
 	NSArray *peopleSorted = [Person findAllSortedByProperty:@"LastName" ascending:YES];
 
+Or, to have the results sorted by multiple properties:
+
+    NSArray *peopleSorted = [Person findAllSortedByProperty:@"LastName,FirstName" ascending:YES];
 
 If you have a unique way of retrieving a single object from your data store, you can get that object directly:
 
@@ -129,6 +132,12 @@ There are also counterpart methods which return NSUInteger rather than NSNumbers
 * countOfEntitiesWithPredicate:(NSPredicate *)
 * countOfEntitiesWithPredicate:(NSPredicate *) inContext:(NSManagedObjectContext *)
 
+#### Aggregate Operations
+
+    NSPredicate *prediate = [NSPredicate predicateWithFormat:@"diaryEntry.date == %@", today];
+    int totalFat = [[CTFoodDiaryEntry aggregateOperation:@"sum:" onAttribute:@"fatColories" withPredicate:predicate] intValue];
+    int fattest  = [[CTFoodDiaryEntry aggregateOperation:@"max:" onAttribute:@"fatColories" withPredicate:predicate] intValue];
+
 #### Finding from a different context
 
 All find, fetch and request methods have an inContext: method parameter

Source/Categories/NSManagedObject+MagicalRecord.m

@@ -737,6 +737,38 @@ - (id) MR_objectWithMinValueFor:(NSString *)property
 	return [self MR_objectWithMinValueFor:property inContext:[self  managedObjectContext]];
 }
 
++ (NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate inContext:(NSManagedObjectContext *)context 
+{
+    NSExpression *ex = [NSExpression expressionForFunction:function 
+                                                 arguments:[NSArray arrayWithObject:[NSExpression expressionForKeyPath:attributeName]]];
+
+    NSExpressionDescription *ed = [[NSExpressionDescription alloc] init];
+    [ed setName:@"result"];
+    [ed setExpression:ex];
+
+    // determine the type of attribute, required to set the expression return type    
+    NSAttributeDescription *attributeDescription = [[[self entityDescription] attributesByName] objectForKey:attributeName];
+    [ed setExpressionResultType:[attributeDescription attributeType]];    
+    NSArray *properties = [NSArray arrayWithObject:ed];
+    MR_RELEASE(ed);
+
+    NSFetchRequest *request = [self requestAllWithPredicate:predicate inContext:context];
+    [request setPropertiesToFetch:properties];
+    [request setResultType:NSDictionaryResultType];    
+
+    NSDictionary *resultsDictionary = [self executeFetchRequestAndReturnFirstObject:request];
+    NSNumber *resultValue = [resultsDictionary objectForKey:@"result"];
+
+    return resultValue;    
+}
+
++ (NSNumber *)aggregateOperation:(NSString *)function onAttribute:(NSString *)attributeName withPredicate:(NSPredicate *)predicate 
+{
+    return [self aggregateOperation:function 
+                        onAttribute:attributeName 
+                      withPredicate:predicate
+                          inContext:[NSManagedObjectContext defaultContext]];    
+}
 
 - (id) MR_inContext:(NSManagedObjectContext *)otherContext
 {