Question

我有两种模式：

class PhonebookEntry {

    private Name name;
    private PhoneNumber phonenumber;


    public PhonebookEntry(Name name, PhoneNumber phonenumber) {
        this.name = name;
        this.phonenumber = phonenumber; 
    }

    public Name getname() {return name;}
    public PhoneNumber getnumber() {return phonenumber;}
    public void call() {

         if (phonenumber.isTollFree()) {
            System.out.println("Dialing (toll-free) "+name+": "+phonenumber);

         }
        else
            System.out.println("Dialing "+ name.getFormal()+ ": ("+phonenumber.getAreaCode() +")"
            + phonenumber.getExchange()+"-" + phonenumber.getLineNumber());
            System.out.println();
    }
    public String tostring() {return name +":"+ phonenumber;}

     public static  PhonebookEntry read(Scanner scanner) {
        if(!scanner.hasNext())  return null;{

         Name name = new Name(scanner.next(), scanner.next());
         PhoneNumber phonenumber = new PhoneNumber(scanner.next());

         return new PhonebookEntry(name,phonenumber);
        }

     }

    public static void main(String[] args) throws FileNotFoundException {
        Scanner scanner = new Scanner(new File("phonebook.text"));
        PhonebookEntry prevEntry =null;
        int count =0;
        PhonebookEntry entry = (read(scanner));
        while(read(scanner) != null) {


             if(prevEntry != null) {
                 if(prevEntry.equals(entry))
                     System.out.println("Duplicate phone book entry"+ '"'+entry.name+": "+ entry.phonenumber+'"'+ "discovered");

                 else if(prevEntry.name.equals(entry.name) && !prevEntry.phonenumber.equals(entry.phonenumber)) {
                     System.out.println("Warning duplicate name encountered " + '"' + entry.name +":"+entry.phonenumber+ '"' + " discovered");

                 }

             }
             count++;
             System.out.println( "phone book entry: "+entry.name + ": "+ entry.phonenumber);
             prevEntry = entry;
            entry.call();

        }

        System.out.println( count+" phonebook entries processed.");

        scanner.close();
    }

}

class Author < ApplicationRecord has_many :books end class Book < ApplicationRecord belongs_to :author end有Book。是否可以在未指定author_id的情况下创建Book，或者是否需要此属性？错误：author_id

Answer 1

默认情况下需要使用rails 5.除非你指定它不是

class Book < ApplicationRecord
  belongs_to :author, optional: true
end

创建对象时是否需要foreign_key列？

1 个答案: