CODEWITHSUNDEEP
google-app-engineentity
Frank
Frank

Reputation: 740

Entity Relationship with Google app engine

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;
}

Upvotes: 0

Views: 259

Answers (1)

tony m
tony m

Reputation: 4779

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.

Upvotes: 1

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