How does many to many relationships work. - ios

Question

Okay I have read apple Core Data Guide. I also look on the web but only found posts about 2 years ago. So I don't know how many to many relationship work. I'm building an app using core data. The app has 3 entities Pokemon, Type, Ability. A Pokemon can have 1 or more type so I set the relationship to to-many. A type can have multiple Pokemon associated with it, so I also should set the relationship to to-many but I don't understand many-to-many relationship. Same goes for the abilities. This is how my data model looks like. There are another 2 entities but I don't really care about those right now.
This is the function I'm using to parse the following API. It grabs the first 20 Pokemon save the name and uses the url to fetch more information about that Pokemon. So I added another task that adds the type and ability to that Pokemon.
private func loadPokemon(url: String) {
let context = coreData.persistentContainer.viewContext
let request = URLRequest(url: URL(string: url)!)
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
if error != nil {
print(error!)
}
do {
let jsonResults = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as! NSDictionary
let pokemonArray = jsonResults.value(forKey: "results") as! [[String: Any]]
for pokemonData in pokemonArray {
guard let name = pokemonData["name"] as? String else {
return
}
guard let pokemonInfoURL = pokemonData["url"] as? String else {
return
}
let pokemon = Pokemon(context: context)
pokemon.name = name
print(1)
self.pokemonMoreInfo(for: pokemon, url: pokemonInfoURL, context: context)
}
}
catch let err {
print(err.localizedDescription)
}
}
task.resume()
}
private func pokemonMoreInfo(for pokemon: Pokemon, url: String, context: NSManagedObjectContext) {
let request = URLRequest(url: URL(string: url)!)
let task = URLSession.shared.dataTask(with: request) { (data, response, error) in
if error != nil {
print(error!)
}
do {
let jsonResults = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as! NSDictionary
//MARK: Pokemon Abilities
if let abilityArray = jsonResults.value(forKey: "abilities") as? [[String: Any]] {
let abilities = pokemon.ability?.mutableCopy() as! NSMutableSet
for abilityData in abilityArray {
guard let abilityDic = abilityData["ability"] as? NSDictionary else {
return
}
let name = abilityDic.value(forKey: "name") as! String
guard let isHidden = abilityData["is_hidden"] as? Bool else {
return
}
guard let slot = abilityData["slot"] as? Int16 else {
return
}
let ability = Ability(context: context)
ability.name = name
ability.isHidden = isHidden
ability.slot = slot
abilities.add(ability)
pokemon.addToAbility(abilities)
}
}
//MARK: Pokemon Type
if let typeArray = jsonResults.value(forKey: "types") as? [[String: Any]] {
let types = pokemon.type?.mutableCopy() as! NSMutableSet
for typeData in typeArray {
guard let typeDic = typeData["type"] as? NSDictionary else {
return
}
let name = typeDic.value(forKey: "name") as! String
guard let slot = typeData["slot"] as? Int16 else {
return
}
let type = Type(context: context)
type.name = name
type.slot = slot
types.add(type)
pokemon.addToType(types)
}
}
}
catch let err {
print(err.localizedDescription)
}
self.coreData.saveContext()
}
task.resume()
}
I'm using this app called SQLight Read-Only. The ability and type are matching to the correct Pokemon. These are screenshots on how my SQLight looks like.
I'm not sure if you guys know about Pokemon, but charizard type is fire and flying and have the abilities of solar-power and blaze. So I know that I'm saving the data correctly. However my SQLight have the same type repeating like fire, grass, poison same goes for the abilities but with the correct Pokemon associated with them. This is how my complete SQLight looks like.
Not sure if it will keep repeating with a many-to-many relationship. So my question is how would I use a many-to-many relationship with Pokemon to type and ability. So how would I add a Pokemon with the same type or same abilities. So later, I can perform a fetch that grabs all Pokemon that have a type of fire or same ability. I'm not sure if I explained my question correctly might be a little confusing.
Would appreciate any help. :)

Answer 1

EDIT:
Actually what I wrote below isn't correct for Core Data. (Thanks for pointing that out in the comments Paulw11.) From the Core Data Guide:
Many-to-Many Relationships
You define a many-to-many relationship using two to-many relationships. The first to-many relationship goes from the first entity (the source entity) to the second entity (the destination). The second to-many relationship goes from the second entity (the original destination entity) to the first entity (the original source entity). You then set each to be the inverse of the other. (If you have a background in database management and this causes you concern, don't worry: if you use an SQLite store, Core Data automatically creates the intermediate join table for you.)
Old answer (incorrect for Core Data):
To create a many-to-many relationship in a relational database you have to add a helper table. In your case you could call it PokemonWithType. It has two columns, one for the Pokemon id and another for the Type id. If you want to find all Types for one specific Pokemon you just query all entries in PokemonWithType with the specific Pokemon id. If you want to find all Pokemon that have a specific Type you do the same just with the Type id.