How to get YapDatabase and Mantle to play nicely with serialization

Question

Suppose I have a model like this:

#import <Mantle/Mantle.h>
#import "MyCustomObject.h"
@interface MyModel : MTLModel <MTLJSONSerializing>
@property (nonatomic, copy, readonly) NSString *UUID;
@property (nonatomic, copy) NSString *someProp;
@property (nonatomic, copy) MyCustomObject *anotherProp;
@end

#import "MyModel.h"
@implementation MyModel
+ (NSDictionary *)JSONKeyPathsByPropertyKey
{
        return @{
            @"UUID": @"id",
            @"anotherProp": NSNull.null
    };
}
}
@end

As you can see, I want to ignore anotherProp during the NSCoding serialization, as well as re-map "UUID" to "id". With YapDatabase, I do a

[transaction setObject:myModelObj forKey:@"key_1" inCollection:@"my_collection"]

but it attempts to serialize anotherProp despite my custom JSONKeyPathsByPropertyKey method, resulting in this error:

*** Caught exception encoding value for key "anotherProp" on class MyModel: -[YapDatabase encodeWithCoder:]: unrecognized selector sent to instance 0xc989630

Do I need to write a custom serializer to get YapDatabase to use JSONKeyPathsByPropertyKey?

Answer 1

Here is my current approach using an MTLModel extension to making this "just work" without a serializer or anything. If there is a serializer implementation or something that would be better, please let me know. Otherwise, this code is a life saver:

// JSONEncodableMTLModel.h
#import <Foundation/Foundation.h>
#import "Mantle.h"

@interface JSONEncodableMTLModel : MTLModel <MTLJSONSerializing>    
+ (NSSet*)propertyKeysToExcludeInDictionaryValue;
@end

//  JSONEncodableMTLModel.m
#import "JSONEncodableMTLModel.h"
#import <objc/runtime.h>

@implementation JSONEncodableMTLModel

+(NSDictionary *)JSONKeyPathsByPropertyKey {
    return @{};
}

+ (NSSet*)propertyKeysToExcludeInDictionaryValue
{
    return [NSSet set];
}

// this is required to ensure we don't have cyclical references when including the parent variable.
+ (NSSet *)propertyKeys {
    NSSet *cachedKeys = objc_getAssociatedObject(self, HSModelCachedPropertyKeysKey);
    if (cachedKeys != nil) return cachedKeys;

    NSMutableSet *keys = [NSMutableSet setWithSet:[super propertyKeys]];

    NSSet *exclusionKeys = [self propertyKeysToExcludeInDictionaryValue];
    NSLog(@"Caching Your Property Keys");
    [exclusionKeys enumerateObjectsUsingBlock:^(NSString *propertyKey, BOOL *stop) {
        if([keys containsObject: propertyKey])
        {
            [keys removeObject: propertyKey];
        }
    }];

    // It doesn't really matter if we replace another thread's work, since we do
    // it atomically and the result should be the same.
    objc_setAssociatedObject(self, HSModelCachedPropertyKeysKey, [keys copy], OBJC_ASSOCIATION_COPY);

    return keys;
}

@end

With this code, I can explicitly set which properties do not need to be serialized by overriding propertyKeysToExcludeInDictionaryValue in any model that is a subclass. Credit goes to Stephen O'Connor.