RestKit – map single source to multiple destinations with relationship mapping - objective-c

Question

Given the following Core Data relationships:
Recipe <<---> Cookbook
Recipe <<---> Author
Cookbook <<---> Author
And this source JSON representing a recipe:
{
"id": 123,
"name": "Recipe Name",
"author": {
"id": 456,
"name": "Author Name"
},
"cookbook": {
"id": 789,
"title": "Cookbook Title"
}
}
I'm looking for a RestKit mapping that will result in a recipe where the following are true:
recipe.cookbook IN recipe.author.cookbooks
recipe.cookbook.author === recipe.author
Here's what I have so far:
recipeMapping =
[RKEntityMapping mappingForEntityForName:#"Recipe" inManagedObjectStore:_store];
[recipeMapping addAttributeMappingsFromDictionary:#{
#"id": #"id",
#"name": #"name"
}];
recipeMapping.identificationAttributes = #[#"id"];
RKEntityMapping *authorMapping =
[RKEntityMapping mappingForEntityForName:#"Author" inManagedObjectStore:_store];
[authorMapping addAttributeMappingsFromDictionary:#{
#"id": #"id",
#"name": #"name"
}];
authorMapping.identificationAttributes = #[#"id"];
RKEntityMapping *cookbookMapping =
[RKEntityMapping mappingForEntityForName:#"Cookbook" inManagedObjectStore:_store];
[cookbookMapping addAttributeMappingsFromDictionary:#{
#"id": #"id",
#"title": #"title"
}];
cookbookMapping.identificationAttributes = #[#"id"];
[recipeMapping
addRelationshipMappingWithSourceKeyPath:#"cookbook"
mapping:cookbookMapping];
[recipeMapping
addRelationshipMappingWithSourceKeyPath:#"author"
mapping:authorMapping];
// Now what?
Upon applying this mapping, recipe.cookbook is a proper instance of Cookbook, and recipe.author is a proper instance of Author, but of course recipe.cookbook.author is nil and recipe.author.cookbooks is empty.

Answer 1

You can't really do this while mapping as you don't have the required information.
You might be able to setup a relationship which uses a foreign key, and rather than have a special attribute for the foreign key use the relationship between the 3 objects instead. I'd say this would be a bit fragile though as it would rely on the order in which the relationships were processed during mapping.
Instead, I would look at doing this either during post-processing (i.e. in the callback after the mapping is complete or by implementing willSave on the managed object subclasses) or on-demand when you actually need to know the relationships. This can be done using KVC, something like:
If you execute a fetch to get all authors, you can build the cookbooks for each using:
author.cookbooks = [author.recipes valueForKeyPath:#"#distinctUnionOfObjects.cookbooks"];