Ability to specify a subset of properties for JSON representation.

Mantle/MTLJSONAdapter.h

@@ -52,6 +52,15 @@
 // to abort parsing (e.g., if the data is invalid).
 + (Class)classForParsingJSONDictionary:(NSDictionary *)JSONDictionary;
 
+// Specifies the subset of properties that will be included in the JSON
+// representation.
+//
+// Subclasses overriding this method should combine their values with those of
+// `super`.
+//
+// Returns an array of property keys.
++ (NSArray *)propertyKeysForJSONRepresentation;
+
 @end
 
 // The domain for errors originating from MTLJSONAdapter.

Mantle/MTLJSONAdapter.m

@@ -244,12 +244,23 @@ - (NSString *)JSONKeyPathForPropertyKey:(NSString *)key {
 
 	id JSONKeyPath = self.JSONKeyPathsByPropertyKey[key];
 	if ([JSONKeyPath isEqual:NSNull.null]) return nil;
-
+	
 	if (JSONKeyPath == nil) {
-		return key;
-	} else {
-		return JSONKeyPath;
+		JSONKeyPath = key;
+	}
+
+	if ([self.modelClass respondsToSelector:@selector(propertyKeysForJSONRepresentation)]) {
+		NSSet *propertyKeys = [NSSet setWithArray:[self.modelClass propertyKeysForJSONRepresentation]];
+
+		if ([propertyKeys containsObject:key]) {
+			return JSONKeyPath;
+		}
+		else {
+			return nil;
+		}
 	}
+
+	return JSONKeyPath;
 }
 
 @end

MantleTests/MTLJSONAdapterSpec.m

@@ -184,4 +184,19 @@
 	expect(error.code).to.equal(MTLJSONAdapterErrorNoClassFound);
 });
 
+it(@"should serialize a subset of properties to JSON", ^{
+	MTLPropertySubsetModel *model = [MTLPropertySubsetModel modelWithDictionary:@{
+		@"name": @"foobar",
+		@"count": @5
+	} error:NULL];
+
+	MTLJSONAdapter *adapter = [[MTLJSONAdapter alloc] initWithModel:model];
+
+	NSDictionary *JSONDictionary = @{
+		@"username": @"foobar"
+	};
+
+	expect(adapter.JSONDictionary).to.equal(JSONDictionary);
+});
+
 SpecEnd

MantleTests/MTLTestModel.h

@@ -55,3 +55,8 @@ extern const NSInteger MTLTestModelNameMissing;
 // Returns a default name of 'foobar' when validateName:error: is invoked
 @interface MTLSelfValidatingModel : MTLValidationModel
 @end
+
+// Specifies a subset of property keys that should be stored in JSON. The subset
+// only contains a single property: "name".
+@interface MTLPropertySubsetModel : MTLTestModel
+@end

MantleTests/MTLTestModel.m

@@ -162,3 +162,11 @@ - (BOOL)validateName:(NSString **)name error:(NSError **)error {
 }
 
 @end
+
+@implementation MTLPropertySubsetModel
+
++ (NSArray *)propertyKeysForJSONRepresentation {
+	return @[ @"name" ];
+}
+
+@end