Mantle property class based on another property? - ios

Question

How can I use Github Mantle to choose a property class based on another property in the same class? (or in the worse case another part of the JSON object).
For example if i have an object like this:
{
"content": {"mention_text": "some text"},
"created_at": 1411750819000,
"id": 600,
"type": "mention"
}
I want to make a transformer like this:
+(NSValueTransformer *)contentJSONTransformer {
return [MTLValueTransformer transformerWithBlock:^id(NSDictionary* contentDict) {
return [MTLJSONAdapter modelOfClass:ETMentionActivityContent.class fromJSONDictionary:contentDict error:nil];
}];
}
But the dictionary passed to the transformer only includes the 'content' piece of the JSON, so I don't have access to the 'type' field. Is there anyway to access the rest of the object? Or somehow base the model class of 'content' on the 'type'?
I have previously been forced to do hack solutions like this:
+(NSValueTransformer *)contentJSONTransformer {
return [MTLValueTransformer transformerWithBlock:^id(NSDictionary* contentDict) {
if (contentDict[#"mention_text"]) {
return [MTLJSONAdapter modelOfClass:ETMentionActivityContent.class fromJSONDictionary:contentDict error:nil];
} else {
return [MTLJSONAdapter modelOfClass:ETActivityContent.class fromJSONDictionary:contentDict error:nil];
}
}];
}

Answer 1

You can pass the type information by modifying the JSONKeyPathsByPropertyKey method:
+ (NSDictionary *)JSONKeyPathsByPropertyKey
{
return #{
NSStringFromSelector(#selector(content)) : #[ #"type", #"content" ],
};
}
Then in contentJSONTransformer, you can access the "type" property:
+ (NSValueTransformer *)contentJSONTransformer
{
return [MTLValueTransformer ...
...
NSString *type = value[#"type"];
id content = value[#"content"];
];
}

Answer 2

I've had a similar issue, and I suspect my solution isn't much better than yours.
I have a common base class for my Mantle objects, and after each is constructed, I call a configure method to give them chance to set up properties that are dependent on more than one "base" (== JSON) property.
Like this:
+(id)entityWithDictionary:(NSDictionary*)dictionary {
NSError* error = nil;
Class derivedClass = [self classWithDictionary:dictionary];
NSAssert(derivedClass,#"entityWithDictionary failed to make derivedClass");
HVEntity* entity = [MTLJSONAdapter modelOfClass:derivedClass fromJSONDictionary:dictionary error:&error];
NSAssert(entity,#"entityWithDictionary failed to make object");
entity.raw = dictionary;
[entity configureWithDictionary:dictionary]; // Give the new entity a chance to set up derived properties
return entity;
}