与Google应用引擎的实体关系 [英] Entity Relationship with Google app engine

问题描述

我正在为使用GAE和Java的大学做一个Web应用程序。我创建了两个实体：用户和汽车。每个用户可以有一辆以上的车辆。所以我创建了一个与car car = new Entity（Car，plate.hashCode（），user.getKey（））; 的关系实体，我的问题是在搜索汽车靠钥匙。我怎样才能做到这一点？因为我的代码 Entity car = getSingleCar（plate）; return always null。这是我的用户代码：

  public static void createOrUpdateCustomer（String name，String surname，
 String phone，String地址，字符串城市，字符串状态，
字符串zip，字符串电子邮件，字符串用户名，字符串密码）{
实体客户= getSingleUser（用户名）; 
 if（customer == null）{
 customer = new Entity（User，username.hashCode（））; 
 customer.setProperty（name，name）; 
 customer.setProperty（surname，姓氏）; 
 customer.setProperty（phone，phone）; 
 customer.setProperty（地址，地址）; 
 customer.setProperty（city，city）; 
 customer.setProperty（state，state）; 
 customer.setProperty（zip，zip）; 
 customer.setProperty（email，email）; 
 customer.setProperty（用户名，用户名）; 
 customer.setProperty（password，password）; 
} else {
 System.out.println（esistegià）; 
 if（name！= null&&！。equals（name））{
 customer.setProperty（firstName，name）; （姓氏）= null&&！。equals（surname））{
 customer.setProperty（lastName，surname）; 
} 
。 （phone）= null&&！。equals（phone））{
 customer.setProperty（phone，phone）; 
} 
 if （地址！= null&&！。equals（address））{
 customer.setProperty（address，address）; 
} 
。 （城市！= null&&！。equals（city））{
 customer.setProperty（city，city）; 
} 
。 （state！= null&&！。equals（state））{
 customer.setProperty（state，state）; 
} 
。 （zip！= null&&！。equals（zip））{
 customer.setProperty（zip，zip）; 
} 
 if （电子邮件！= null&&！。equals（email））{
 customer.setProperty（email，email）; 
} 
。 （用户名！= null&&！。equals（username））{
 customer.setProperty（username，username）; 
} 
。 （密码！= null&&！。equals（password））{
 customer.setProperty（password，password）; 
} 
。 
} 
} 
 Util.persistEntity（customer）; 
} 
 
 public static Entity getSingleUser（String username）{
 Key key = KeyFactory.createKey（User，username.hashCode（））; 
返回Util.findEntity（key）; 
} 
 
 public static Iterable< Entity> getUser（字符串用户名）{
 Iterable< Entity>实体= Util.listEntities（用户，用户名，用户名）; 
返回实体; 
} 
 
 public static Iterable< Entity> getUserById（String id）{
 Iterable< Entity> entities = Util.listEntities（User，ID，id）; 
返回实体; 
} 
 
 
 public static Iterable< Entity> getAllUsers（）{
 Iterable< Entity> entities = Util.listEntities（User，null，null）; 
返回实体; 
} 
  

创建Car的代码：

  public static Entity createOrUpdateCar（String username，String plate，String model，String revision，String service，String brand，String motor）{
 Entity user = User.getSingleUser（用户名）; 
 System.out.println（user.getKey（））; 
实体车= getSingleCar（板）; 
 
 if（car == null&& user！= null）{
 System.out.println（ènull）; 
 car = new实体（Car，plate.hashCode（），user.getKey（））; 
 car.setProperty（盘子，盘子）; 
 car.setProperty（user，username）; 
 car.setProperty（model，model）; 
 car.setProperty（revision，revision）; 
 car.setProperty（service，service）; 
 car.setProperty（mechanic，null）; 
 car.setProperty（品牌，品牌）; 
 car.setProperty（电机，电机）; 
 car.setProperty（riparazione，null）; 
} else {
 System.out.println（id+ car.getProperty（plate））; 
} 
 Util.persistEntity（car）; 
回车; 
} 
 
 public static Entity getSingleCar（String plate）{
 Key key = KeyFactory.createKey（Car，plate.hashCode（））; 
返回Util.findEntity（key）; 
} 
 
 public static Entity getSingleCarByKey（Key key）{
 return Util.findEntity（key）; 
} 
 
 public static Iterable< Entity> getCar（String plate）{
 Iterable< Entity> entities = Util.listEntities（Car，plate，plate）; 
返回实体; 
} 
 
 public static Iterable< Entity> getAllCars（）{
 Iterable< Entity>实体= Util.listEntities（Car，null，null）; 
返回实体; 
} 
  

解决方案

发现你的代码总是为空的原因 Entity car = getSingleCar（plate）;是你在使用这个查询的错误键。

创建一个新的Car实体时，使用键（Car，plate。 hashCode（），user.getKey（））; 它有3个部分 - kind，id，祖先关键字

但是在查询汽车时，你没有添加祖先密钥，因此你最终使用一个不匹配的密钥进行查询 Key key = KeyFactory.createKey（Car，plate.hashCode（））; Key key = KeyFactory.createKey（user.getKey（）， Car，plate.hashCode（））;
请更改您的查询方法以获取此信息。 阅读此类似文章以更好地理解实体构造函数详细信息和KeyFactory方法。

I'm doing a web application for university with GAE and Java. I created two entity: User and Car. Each user can have plus than one car. So i created a relationship entity with car car= new Entity("Car",plate.hashCode(), user.getKey()); My problem is in the searching of the car by key. How can i do this? because my code Entity car= getSingleCar(plate); return always null. This is my code for User:

public static void createOrUpdateCustomer(String name, String surname,
        String phone, String address, String city, String state,
        String zip, String email, String username, String password) {
    Entity customer = getSingleUser(username);
    if (customer == null) {
        customer = new Entity("User", username.hashCode());
        customer.setProperty("name", name);
        customer.setProperty("surname", surname);
        customer.setProperty("phone", phone);
        customer.setProperty("address", address);
        customer.setProperty("city", city);
        customer.setProperty("state", state);
        customer.setProperty("zip", zip);
        customer.setProperty("email", email);
        customer.setProperty("username", username);
        customer.setProperty("password", password);
    } else {
        System.out.println("esiste già");
        if (name != null && !"".equals(name)) {
            customer.setProperty("firstName", name);
        }
        if (surname != null && !"".equals(surname)) {
            customer.setProperty("lastName", surname);
        }
        if (phone != null && !"".equals(phone)) {
            customer.setProperty("phone", phone);
        }
        if (address != null && !"".equals(address)) {
            customer.setProperty("address", address);
        }
        if (city != null && !"".equals(city)) {
            customer.setProperty("city", city);
        }
        if (state != null && !"".equals(state)) {
            customer.setProperty("state", state);
        }
        if (zip != null && !"".equals(zip)) {
            customer.setProperty("zip", zip);
        }
        if (email != null && !"".equals(email)) {
            customer.setProperty("email", email);
        }
        if (username != null && !"".equals(username)) {
            customer.setProperty("username", username);
        }
        if (password != null && !"".equals(password)) {
            customer.setProperty("password", password);
        }
    }
    Util.persistEntity(customer);
}

public static Entity getSingleUser(String username) {
    Key key = KeyFactory.createKey("User", username.hashCode());
    return Util.findEntity(key);
}

public static Iterable<Entity> getUser(String username) {
    Iterable<Entity> entities = Util.listEntities("User", "username", username);
    return entities;
}

public static Iterable<Entity> getUserById(String id) {
    Iterable<Entity> entities = Util.listEntities("User", "ID", id);
    return entities;
}


public static Iterable<Entity> getAllUsers() {
    Iterable<Entity> entities = Util.listEntities("User", null, null);
    return entities;
}

e the code for create a Car:

public static Entity createOrUpdateCar(String username, String plate, String model, String revision, String service, String brand, String motor){
    Entity user= User.getSingleUser(username);
    System.out.println(user.getKey());
    Entity car= getSingleCar(plate);

    if(car==null && user!=null){
        System.out.println("è null");
        car= new Entity("Car",plate.hashCode(), user.getKey());
        car.setProperty("plate", plate);
        car.setProperty("user", username);
        car.setProperty("model", model);
        car.setProperty("revision", revision);
        car.setProperty("service", service);
        car.setProperty("mechanic", null);
        car.setProperty("brand", brand);
        car.setProperty("motor", motor);
        car.setProperty("riparazione", null);
    }else {
        System.out.println("id"+car.getProperty("plate"));
    }
    Util.persistEntity(car);
    return car;
}

public static Entity getSingleCar(String plate) {
    Key key = KeyFactory.createKey("Car", plate.hashCode());
    return Util.findEntity(key);
}

public static Entity getSingleCarByKey(Key key) {
    return Util.findEntity(key);
}

public static Iterable<Entity> getCar(String plate) {
    Iterable<Entity> entities = Util.listEntities("Car", "plate", plate);
    return entities;
}

public static Iterable<Entity> getAllCars() {
    Iterable<Entity> entities = Util.listEntities("Car", null, null);
    return entities;
}

解决方案

After seeing your code, I find that the reason why you are always getting null for your code Entity car= getSingleCar(plate); is that you are using the wrong key for this query.

When creating a new Car entity you use the key ("Car",plate.hashCode(), user.getKey()); which has 3 parts - kind, id, ancestor key

But while querying for car, you are not adding the ancestor key and hence you end up using a non-matching key for the query Key key = KeyFactory.createKey("Car", plate.hashCode());

Instead you should be using a key which also has the ancestor key Key key = KeyFactory.createKey(user.getKey(), "Car", plate.hashCode()); Please change your query method to have this. Read this similar post to understand entity constructor details and KeyFactory methods better.

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