init(with resolver: Resolver) throws

class CategoryPlainObject: Mappable {
    
    let id: Int64
    let name: String
    
    var positions: [PositionPlainObject] = [] // Array of objects
    
    required init(with resolver: Resolver) throws {
        
        self.id        = try resolver.value("id")
        self.name      = try resolver.value("name")
        self.positions = try resolver.value("positions")
    }
}

class PositionPlainObject: Mappable {
    
    let id: Int64
    let name: String
    let price: Double
    
    var category: CategoryPlainObject? = nil // Nested object
    
    required init(with resolver: Resolver) throws {
        
        self.id       = try  resolver.value("id")
        self.name     = try  resolver.value("name")
        self.price    = try  resolver.value("price")
        self.category = try? resolver.value("category")
    }
}

struct User: Mappable {
    
    let id: Int64
    let name: String
    
    init(with resolver: Resolver) throws {
        
        self.id   = try resolver.value("id")
        self.name = try resolver.value("name")
    }
}

/// A simple protocol for a nice exmaple

protocol Storable: class {
    
    static var entityName: String { get }
}

extension Storable where Self: NSManagedObject {
    
    static var entityName: String {
        
        return NSStringFromClass(self).components(separatedBy: ".").last ?? ""
    }
    
    init(in context: NSManagedObjectContext) {
        
        guard let entity = NSEntityDescription.entity(forEntityName: Self.entityName, in: context) else {
            
            fatalError("Cannot create entity for entity name: \(self.entityName)")
        }
        
        self.init(entity: entity, insertInto: context)
    }
}

class CategoryModelObject: NSManagedObject, Storable {
    
    @NSManaged var id: Int64
    @NSManaged var name: String
    @NSManaged var positions: NSSet
}

class PositionModelObject: NSManagedObject, Storable {
    
    @NSManaged var id: Int64
    @NSManaged var name: String
    @NSManaged var price: Double
    @NSManaged var category: CategoryModelObject
}

let categoryModelObject = CategoryModelObject(in: context)
let positionModelObject = PositionModelObject(in: context)

/// Fill some data to categoryModelObject & positionModelObject

...

/// And mapping from CoreData objects

let transformer = Transformer(from: .coredata)

let categoryPlainObject: CategoryPlainObject = try transformer.transform(from: categoryModelObject)
let positionPlainObject: PositionPlainObject = try transformer.transform(from: positionModelObject)