Core Data Cross Reference Table (Swift 4) - swift

Question

Imagine I have an app with two Core Data entities: ingredients and cookies.
Ingredients have a title, a type, an image, and a few other properties.
Cookies have a title, a type, an image and a few other properties.
A cookie is comprised of several ingredients. Ingredients can be used in multiple different cookies. The Cookie entity and the Ingredient entity have an inverse relationship which allows me to see which cookies have which ingredients and which ingredients are used in which cookies.
All of this is working fine.
Here is my problem:
In addition to several other ingredients of varying quantities, chocolate chip cookies require one egg but oatmeal raisin cookies require two eggs.
If I was designing a MySQL database to model all this, I'd have a Cookie table (similar to my Cookie Core Data entity), an Ingredient table (similar to my Ingredient Core Data entity), and a cross-reference table CookieIngredient, which would include a cookieID, an ingredientID, and a quantity.
I've tried to do this in Core Data, but I can't seem to get the entity for the cross-reference table created correctly. I create an Entity named CookieIngredient with Integer 16 fields of cookieID, ingredientID, and quantity, but then when I attempt to create an NSManagedObject Subclass, the class which is created -- unlike the other two classes -- displays numerous errors telling me "extensions cannot contain stored properties", which doesn't seem to have anything to do with anything else in the universe.
None of the many "recipe" Swift tutorials I've found online use Core Data, and the ones that do don't use a cross-reference table.
Any suggestions appreciated. (Am I doing this the wrong way?)
do{
let fetchRequest : NSFetchRequest<Ingredient> = Ingredient.createFetchRequest()
fetchRequest.predicate = NSPredicate(format: "title == %#", title)
let fetchedResults = try the_context.fetch(fetchRequest)
if let anIngredient = fetchedResults.first {
let the_recipe = CookieIngredient(context: the_context)
the_recipe.quantity = amount
the_recipe.unit = unit
the_recipe.mutableSetValue(forKey: "recipeIngredient").add(anIngredient)
the_recipe.mutableSetValue(forKey: "recipeCookie").add(Cookie)
}
}
catch{
print( "unable to update ingredients" )
}
Assuming I have a Cookie object, I should be able to do this. But the error I'm getting is:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSManagedObjects of entity 'CookieIngredient' do not support -mutableSetValueForKey: for the property 'recipeIngredient''
Is mutableSetValue not the correct way to create the relationship?
Note: mutableSetValue was not the correct way to do it.
extension CookieIngredient {
#nonobjc public class func createfetchRequest() -> NSFetchRequest<CookieIngredient> {
return NSFetchRequest<CookieIngredient>(entityName: "CookieIngredient")
}
#NSManaged public var quantity: String
#NSManaged public var unit: String
#NSManaged public var recipeCookie: Cookie
#NSManaged public var recipeIngredient: Ingredient
}