Fix incorrect type transformation when the destination class is a mutable class. fixes #1201

Code/ObjectMapping/RKMappingOperation.m

@@ -65,6 +65,19 @@ static BOOL RKIsManagedObject(id object)
     return managedObjectClass && [object isKindOfClass:managedObjectClass];
 }
 
+/**
+ NOTE: Because most Foundation classes are implemented as class-clusters, we cannot introspect them for mutability. Instead, we evaluate the destination type and if it is a mutable class, we return `YES` to trigger an invocation of `mutableCopy`.
+ */
+static BOOL RKIsMutableTypeTransformation(id value, Class destinationType)
+{
+    if ([destinationType isEqual:[NSMutableArray class]]) return YES;
+    else if ([destinationType isEqual:[NSMutableDictionary class]]) return YES;
+    else if ([destinationType isEqual:[NSMutableString class]]) return YES;
+    else if ([destinationType isEqual:[NSMutableSet class]]) return YES;
+    else if ([destinationType isEqual:[NSMutableOrderedSet class]]) return YES;
+    else return NO;
+}
+
 id RKTransformedValueWithClass(id value, Class destinationType, NSValueTransformer *dateToStringValueTransformer);
 id RKTransformedValueWithClass(id value, Class destinationType, NSValueTransformer *dateToStringValueTransformer)
 {
@@ -78,6 +91,8 @@ id RKTransformedValueWithClass(id value, Class destinationType, NSValueTransform
     } else if (RKClassIsCollection(destinationType) && !RKObjectIsCollection(value)) {
         // Call ourself recursively with an array value to transform as appropriate
         return RKTransformedValueWithClass(@[ value ], destinationType, dateToStringValueTransformer);
+    } else if (RKIsMutableTypeTransformation(value, destinationType)) {
+        return [value mutableCopy];
     } else if ([sourceType isSubclassOfClass:[NSString class]] && [destinationType isSubclassOfClass:[NSDate class]]) {
         // String -> Date
         return [dateToStringValueTransformer transformedValue:value];

Tests/Logic/ObjectMapping/RKObjectMappingNextGenTest.m

@@ -1429,6 +1429,23 @@
     assertThat(user.favoriteColors, is(equalTo(@[ @"Blue" ])));
 }
 
+- (void)testMappingArrayToMutableArray
+{
+    RKObjectMapping *mapping = [RKObjectMapping mappingForClass:[RKTestUser class]];
+    [mapping addAttributeMappingsFromDictionary:@{ @"favoriteColors": @"mutableFavoriteColors" }];
+    
+    NSDictionary *dictionary = @{ @"favoriteColors": @[ @"Blue", @"Red" ] };
+    RKTestUser *user = [RKTestUser user];
+    RKMappingOperation *operation = [[RKMappingOperation alloc] initWithSourceObject:dictionary destinationObject:user mapping:mapping];
+    RKObjectMappingOperationDataSource *dataSource = [RKObjectMappingOperationDataSource new];
+    operation.dataSource = dataSource;
+    NSError *error = nil;
+    [operation performMapping:&error];
+    
+    assertThat(user.mutableFavoriteColors, is(equalTo(@[ @"Blue", @"Red" ])));
+    assertThat(user.mutableFavoriteColors, is(instanceOf([NSMutableArray class])));
+}
+
 - (void)testMappingASingularValueToASet
 {
     RKObjectMapping *mapping = [RKObjectMapping mappingForClass:[RKTestUser class]];

Tests/Models/RKTestUser.h

@@ -23,6 +23,7 @@
 @property (nonatomic, strong) NSDate *birthDate;
 @property (nonatomic, strong) NSDate *favoriteDate;
 @property (nonatomic, strong) NSArray *favoriteColors;
+@property (nonatomic, strong) NSMutableArray *mutableFavoriteColors;
 @property (nonatomic, strong) NSDictionary *addressDictionary;
 @property (nonatomic, strong) NSURL *website;
 @property (nonatomic, strong) NSNumber *isDeveloper;