How to select a single Entity by key (low level java datastore API)

问题

I'm a bit confused on how to create a "Key" object to select exactly 1 row of my Entity ("Customer").

My code :

Query query = new Query("Customer");
// **** how do I have to create this key ???
Key key = KeyFactory.createKey("Customer", 1);
// ****
FilterPredicate keyFilter =  new FilterPredicate(Entity.KEY_RESERVED_PROPERTY,       FilterOperator.EQUAL, key);
query.setFilter(keyFilter);
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
PreparedQuery pq = datastore.prepare(query);
Entity customer = pq.asSingleEntity();
if  (! (customer == null)) {
    log.info("OK !");
}
else {
    log.info("no customer found");
}

My result is always : "no customer found".

With the datastore viewer (I'm working locally) I can see 3 rows :

• id/name = 1 email = "your email" name = "your name"
• id/name = 2 email = "your email" name = "your name"
• id/name = 3 email = "your email" name = "your name"

I want to select customer with id/name=1. I've tried : KeyFactory.createKey("Customer", 1); and KeyFactory.createKey("Customer", "your name");

but with no success.

When I do programmatically a search (asList) on "Customer" and I print the keys' values I see :

• row 1 key = Customer("your name")/Customer(1)
• row 2 key = Customer("your name")/Customer(2)
• row 3 key = Customer("your name")/Customer(3)

The printable values of the keys are in the format : "Entity(name)/Entity(id/name)"

How can I create such key values in my code ? On the javadoc I see only :

• createKey("kind", id)
• createKey("kind", "name")

The other methods require an ancestor, I've not created an ancestor .... This is the code that I've executed to created the 3 rows (it has been executed 3 times) :

Key customerKey = KeyFactory.createKey("Customer", "your name");
Entity customer = new Entity("Customer", customerKey);
customer.setProperty("name", "your name");
customer.setProperty("email", "your email");
DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
datastore.put(customer);

Thanks in advance !

Regards

Max

In other words, how do create a Key whose "toString" gives the following result ?

• Code : log.info("customer KeyFactory.keyToString = "+KeyFactory.keyToString(key));
• Output : INFO: customer KeyFactory.keyToString = Customer("your name")/Customer(1)

回答1:

The creation of the Entities is not correct. You dont have to create a key on your own. If you create a Entity with a kind and keyname or ID parameter, a key will be created automaticlly out of the kind and the keyname or ID.

Entity customer = new Entity("Customer", "your name");
customer.setProperty("email", "your email");

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
datastore.put(customer);

That is all you need to create the Entity you want.

When you give a kind and a key, a child Entity will be created. So you had created a Customer with "your name" who is a Customer with the ID 1.

To get the Entity with a key you just have to take the same parameters to create a key.

Key key = KeyFactory.createKey("Customer", "your name");

I would prefer to use get(), but i dont know what you want to do. For Example to get the email of a Customer:

    Entity e;
    try {
        e = datastore.get(key);

        String myEmail= (String) e.getProperty("your email");

回答2:

I think you want to use get() not query() when you have a Key.

https://developers.google.com/appengine/docs/java/javadoc/com/google/appengine/api/datastore/DatastoreService#get(com.google.appengine.api.datastore.Key)

回答3:

Let us assume that you have a Java entity class called Product and you wish to retrieve a unique record given its key i.e. ID

The method would look something like this:

    public Product getProduct(Long id) {

        //Get Persistence Manager
        PersistenceManager pm = ...;

        Key k = KeyFactory.createKey(Product.class.getSimpleName(), id);
        Product product = null;
        try {
           product = pm.getObjectById(Product.class, k);
        } 
        finally {
           pm.close();
        }

    return product;
}

So in your example, I think you can replace Product with your class name. I guess your class name is Customer

来源：https://stackoverflow.com/questions/14368995/how-to-select-a-single-entity-by-key-low-level-java-datastore-api