employee可以拥有多个animal，但animal必须只有一个employee。
首先，我添加了一个雇员。然后在添加动物时，我选择了雇员。选择了雇员后，我按下了保存按钮。我想将我刚刚添加的动物转移到该雇员。我想将雇员的id添加到我添加的动物的雇员属性中。

动物模型：

struct Animal {
    let id: String?
    var name: String?
    var sound: String?
    var age: Double?
    var waterConsumption: Double?
    var employee: Employee?
    var animalType: String?
}

员工模型：

struct Employee {
    let id: String?
    var name: String?
    var lastName: String?
    var age: Int?
    var gender: String?
    var salary: Double?
    var experienceYear: String?
    var animals: [Animal]?
}

Zookeeper：
错误在此行：新动物.雇员？.动物？.append（新动物）Overlapping accesses to 'newAnimal.employee', but modification requires exclusive access; consider copying to a local variable

protocol ZooManagerProtocol {
    var animals: [Animal] { get set }
    var employees: [Employee] { get set }
    
    func addNewAnimal(_ model: Animal)
}

class ZooManager: ZooManagerProtocol {
    static var shared = ZooManager()
    
    var animals: [Animal] = []
    var employees: [Employee] = []
    
    private init() { }
    
func addNewAnimal(_ model: Animal) {
    var newAnimal = model
    guard var employee = employees.filter({ $0.id == model.employee?.id }).first else { return }
    newAnimal.employee = employee
    newAnimal.employee?.animals?.append(newAnimal)
    dump(newAnimal)
}
    
    func addNewEmployee(_ model: Employee) {
        employees.append(model)
    }
}