I have a `class` that has a reference to another `class`. A student class has a variable that has a reference to module info. I want to insert the `moduleId` into an array but the struct is made up of different types of objects and is of type `moduleRef`. I would like to get the `moduleId` which is of type string. How can I grab this moduleId element form `ModuleRef`.

```
class Student {
    var firstName: String
    var lastName: String
    var modulesRefs: [ModuleRef]

    var fullName: String {
        return self.firstName + " " + self.lastName
    }

    init(firstName: String, lastName: String, modulesRefs: [ModuleRef]) {
        self.firstName = firstName
        self.lastName = lastName
        self.modulesRefs = modulesRefs
    }

    func addModuleId(_ moduleId: String) {
        self.modulesRefs.insert(moduleId, at: 0)
    }

     func removeModuleId(_ moduleId: String) {
        self.modulesRefs = self.modulesRefs.filter { $0.moduleId != moduleId }
    }
}

class ModuleRef {
    var moduleName: String
    var moduleId: String
    var moduleStartdate: Int

    init(moduleName: String, moduleId: String, moduleStartdate: Int) {
        self.moduleName = moduleName
        self.moduleId = moduleId
        self.moduleStartdate = moduleStartdate
    }
}

```

> 
> Cannot convert value of type 'String' to expected argument type 'ModuleRef'
> 
> 
>