Unrecognized selector sent to instance: key-value pair - objective-c

Question

I get
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSDictionaryM metadata]: unrecognized selector sent to instance 0xd0b6250'
when i reach and execute this part of my code:
IDTherapyMetadata* meta = [self.dataCon getMetaDataFromId:content.metadata];
the method in dataCon looks like this:
-(IDTherapyMetadata*)getMetaDataFromId:(NSString*)metaDataId
{
for (IDTherapyMetadata* metadata in self.parser.metadata)
{
if([metaDataId compare:metadata.objId] == NSOrderedSame)
{
return metadata;
}
}
return nil;
}
The strange thing is, if I write
IDTherapyMetadata* meta = [self.dataCon getMetaDataFromId:[content valueForKey:#"metadata"]];
instead it works just fine.
However I want to be able to use dotnotation and looking through apple's documentation
https://developer.apple.com/library/ios/documentation/cocoa/conceptual/KeyValueCoding/Articles/BasicPrinciples.html
haven't helped me.
Why i'm I getting this behaviour and how do I fix it?
Any help is greatly appreciated
EDIT:
The code for IDTherapyContents.h:
#import <Foundation/Foundation.h>
#interface IDTherapyContents : NSObject
#property (nonatomic, copy)NSString* urlId;
#property (nonatomic, copy)NSString* url;
#property (nonatomic, copy)NSString* type;
#property (nonatomic, copy)NSString* timestamp;
#property (nonatomic, copy)NSString* metadata;
#property (nonatomic, copy)NSString* lang;
#property (nonatomic, copy)NSString* objId;
//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;
#end
Code for IDTherpayMetaData.h
#import <Foundation/Foundation.h>
#import "IDTherapyImages.h"
#interface IDTherapyMetadata : NSObject
//Array contains 'String' objects
#property (nonatomic, strong)NSMutableArray* thumbnails;
#property (nonatomic, copy)NSString* title;
#property (nonatomic, copy)NSString* timestamp;
#property (nonatomic, copy)NSString* subCategory;
#property (nonatomic, copy)NSString* lang;
#property (nonatomic, copy)NSString* objId;
#property (nonatomic, copy)NSString* description5;
#property (nonatomic, copy)NSString* description4;
#property (nonatomic, copy)NSString* description3;
#property (nonatomic, copy)NSString* description2;
#property (nonatomic, copy)NSString* description1;
#property (nonatomic, copy)NSString* category;
//Array contains 'IDTherapyImages' objects
#property (nonatomic, strong)NSMutableArray* images;
//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;
#end
Code for IDTherapyProducts.h:
#interface IDTherapyProducts : NSObject
//Array contains 'String' objects
#property (nonatomic, strong)NSMutableArray* contents;
#property (nonatomic, copy)NSString* uniqueId;
#property (nonatomic, copy)NSString* timestamp;
#property (nonatomic, copy)NSString* price;
#property (nonatomic, copy)NSString* posted;
#property (nonatomic, copy)NSString* normalPrice;
#property (nonatomic, copy)NSString* metadata;
#property (nonatomic, copy)NSString* lang;
#property (nonatomic, copy)NSString* objId;
#property (nonatomic, copy)NSString* badge;
//methods
-(id)initWithJsonDic:(NSDictionary*)jsonDic;
#end

Answer 1

This is a simple case of confusing properties with KVC. They are different things.
content.metadata is nothing more than syntactic sugar for [content metadata]. So you need a method on content called -metadata and as content is an NSDictionary that method doesn't exist. So that's why the exception is thrown.
On any normal object [content valueForKey: #"metadata"] would also throw an exception because KVC looks for a method with that name. However, on NSDictionary -valueForKey: is overridden to call -objectForKey: unless the key starts with an # in which case the # is stripped and [super valueForKey: ...] is invoked.
However I want to be able to use dotnotation
Then you need a method on NSDictionary called -metadata. You could use a category.
#interface NSDictionary(MyMetadataExtension)
-(id) metadata;
#end
#implementation NSDictionary(MyMetadataExtension)
-(id) metadata
{
return [self objectForKey: #"metadata"];
}
#end