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Programming Google App Engine

Cover of Programming Google App Engine by Dan Sanderson Published by O'Reilly Media, Inc.
  1. Programming Google App Engine
    1. SPECIAL OFFER: Upgrade this ebook with O’Reilly
    2. Preface
      1. Using This Book
      2. Conventions Used in This Book
      3. Using Code Samples
      4. Safari® Books Online
      5. How to Contact Us
      6. Acknowledgments
    3. 1. Introducing Google App Engine
      1. The Runtime Environment
      2. The Static File Servers
      3. The Datastore
      4. The Services
      5. Google Accounts
      6. Task Queues and Cron Jobs
      7. Developer Tools
      8. The Administration Console
      9. Things App Engine Doesn’t Do...Yet
      10. Getting Started
    4. 2. Creating an Application
      1. Setting Up the SDK
      2. Developing the Application
      3. Registering the Application
      4. Uploading the Application
      5. Introducing the Administration Console
    5. 3. Handling Web Requests
      1. The App Engine Architecture
      2. Configuring the Frontend
      3. How the App Is Run
      4. Quotas and Limits
    6. 4. Datastore Entities
      1. Entities, Keys, and Properties
      2. Introducing the Python Datastore API
      3. Introducing the Java Datastore API
      4. Property Values
      5. Keys and Key Objects
      6. Using Entities
    7. 5. Datastore Queries
      1. Queries and Kinds
      2. Query Results and Keys
      3. GQL
      4. The Python Query API
      5. The Java Query API
      6. Introducing Indexes
      7. Automatic Indexes and Simple Queries
      8. Custom Indexes and Complex Queries
      9. Not-Equal and IN Filters
      10. Unset and Nonindexed Properties
      11. Sort Orders and Value Types
      12. Queries and Multivalued Properties
      13. Configuring Indexes
    8. 6. Datastore Transactions
      1. Entities and Entity Groups
      2. What Can Happen in a Transaction
      3. Transactions in Python
      4. Transactions in Java
      5. How Entities Are Updated
      6. How Entities Are Read
      7. Batch Updates
      8. How Indexes Are Updated
    9. 7. Data Modeling with Python
      1. Models and Properties
      2. Property Declarations
      3. Modeling Relationships
      4. Model Inheritance
      5. Queries and PolyModels
      6. Creating Your Own Property Classes
    10. 8. The Java Persistence API
      1. Setting Up JPA
      2. Entities and Keys
      3. Entity Properties
      4. Embedded Objects
      5. Saving, Fetching, and Deleting Objects
      6. Transactions in JPA
      7. Queries and JPQL
      8. Relationships
      9. For More Information
    11. 9. The Memory Cache
      1. The Python Memcache API
      2. The Java Memcache API
    12. 10. Fetching URLs and Web Resources
      1. Fetching URLs in Python
      2. Fetching URLs in Java
      3. Asynchronous Requests in Python
    13. 11. Sending and Receiving Mail and Instant Messages
      1. Enabling Inbound Services
      2. Sending Email Messages
      3. Receiving Email Messages
      4. Sending XMPP Messages
      5. Receiving XMPP Messages
    14. 12. Bulk Data Operations and Remote Access
      1. Setting Up the Remote API for Python
      2. Setting Up the Remote API for Java
      3. Using the Bulk Loader Tool
      4. Using the Remote Shell Tool
      5. Using the Remote API from a Script
    15. 13. Task Queues and Scheduled Tasks
      1. Task Queues
      2. Scheduled Tasks
    16. 14. The Django Web Application Framework
      1. Installing Django
      2. Creating a Django Project
      3. The Request Handler Script
      4. The Django App Engine Helper
      5. Creating a Django Application
      6. Using App Engine Models With Django
      7. Using Django Unit Tests and Fixtures
      8. Using Django Forms
    17. 15. Deploying and Managing Applications
      1. Uploading an Application
      2. Using Versions
      3. Managing Service Configuration
      4. Managing Indexes
      5. Browsing and Downloading Logs
      6. Inspecting the Datastore
      7. Application Settings
      8. Managing Developers
      9. Quotas and Billing
      10. Getting Help
    18. Index
    19. About the Author
    20. Colophon
    21. SPECIAL OFFER: Upgrade this ebook with O’Reilly

Using Entities

Let’s look briefly at how to retrieve entities from the datastore using keys, how to inspect the contents of entities, and how to update and delete entities. The API methods for these features are straightforward.

Getting Entities Using Keys

Given a complete key for an entity, you can retrieve the entity from the datastore.

In the Python API, you can call the get() function in the db package with the Key object as an argument:

from google.appengine.ext import db

k = db.Key('Entity', 'alphabeta')

e = db.get(k)

If you know the kind of the entity you are fetching, you can also use the get() class method on the appropriate entity class. This does a bit of type checking, ensuring that the key you provide is of the appropriate kind:

class Entity(db.Expando):

e = Entity.get(k)

To fetch multiple entities in a batch, you can pass the keys to get() as a list. Given a list, the method returns a list containing entity objects, with None values for keys that do not have a corresponding entity in the datastore.

entities = db.get([k1, k2, k3])

Getting a batch of entities in this way performs a single service call to the datastore for the entire batch. This is faster than getting each entity in a separate call. It is also subject to the service call size limit (one megabyte), so make sure the total size of all of the entities will not exceed this limit when doing a batch get. Batch gets are also subject to an entity count limit.

For convenience, entity classes include methods that take just the IDs or key names and retrieve the corresponding entities, inferring the kind from the class name. See get_by_id() and get_by_key_name() in the official reference documentation.

In the Java low-level API, you get an entity by its key using a DatastoreService instance (returned by DatastoreServiceFactory.getDatastoreService()). The instance provides a get() method that takes a Key for a single entity get, or an Iterable<Key> for a batch get. If given an iterable of keys, get() returns a Map of Key to Entity:

DatastoreService ds = DatastoreServiceFactory.getDatastoreService();

Map<Key, Entity> entities = ds.get(new ArrayList(Arrays.asList(k1, k2, k3)));

Entity e1 = entities.get(k1);

Of course, you won’t always have the keys for the entities you want to fetch from the datastore. To retrieve entities that meet other criteria, you use datastore queries. Queries are discussed in Chapter 5.

Inspecting Entity Objects

Entity objects have methods for inspecting various aspects of the entity.

In the Java API, the methods of the Entity class provide straightforward access to the key (getKey()) and kind (getKind()) of the entity. The getProperty() method returns the value of a property given its name. The hasProperty() method tests whether a property is set. setProperty() takes a name and a value and sets the property, replacing any existing value.

The Python API has several features for inspecting entities worth mentioning here. You’ve already seen the key() method of an entity object, which returns the db.Key.

The is_saved() method returns False if the object has not been saved to the datastore since the object was constructed. If the object has been saved since it was constructed, or if the object was retrieved from the datastore, the method returns True. The method continues to return True even if the object’s properties have been modified, so do not rely on this method to track changes to properties of previously saved entities.

e = Entity()
# e.is_saved() == False

# e.is_saved() == True

The Java API does not have an equivalent to is_saved().

In Python, entity properties can be accessed and modified just like object attributes:

e.prop1 = 1
e.prop2 = 'two'

print "prop2 has the value " + e.prop2

You can use Python built-in functions for accessing object attributes to access entity properties. For instance, to test that an entity has a property with a given name, use the hasattr() built-in:

if hasattr(e, 'prop1'):
    # ...

To get or set a property whose name is a string, use getattr() and setattr(), respectively:

# Set prop1, prop2, ..., prop9.
for n in range(1, 10):
    value = n * n
    setattr(e, 'prop' + str(n), value)

value = getattr(e, 'prop' + str(7))

While entity objects support accessing properties using these methods, the objects do not actually store property values as object attributes. For instance, you cannot use Python’s dir() built-in to get a list of an entity’s properties. Instead, entity objects provide their own method, instance_properties(), for this purpose:

for name in e.instance_properties():
    value = getattr(e, name)

Saving Entities

In Python, calling the put() method on an entity object saves the entity to the datastore. If the entity does not yet exist in the datastore, put() creates the entity. If the entity exists, put() updates the entity so that it matches the object.

e = Entity()
e.prop = 123


When you update an entity, the app sends the complete contents of the entity to the datastore. The update is all or nothing: there is no way to send just the properties that have changed to the datastore. There is also no way to update a property on an entity without retrieving the complete entity, making the change, then sending the new entity back.

You use the same API to create an entity as you do to update an entity. The datastore does not make a distinction between creates and updates. If you save an entity with a complete key (such as a key with a kind and a key name) and an entity already exists with that key, the datastore replaces the existing entity with the new one.


If you want to test that an entity with a given key does not exist before you create it, you can do so using the transaction API. You must use a transaction to ensure that another process doesn’t create an entity with that key after you test for it and before you create it. For more information on transactions, see Chapter 6.

If you have several entity objects to save, you can save them all in one call using the put() function in the db package. The put() function can also take a single entity object.

db.put([e1, e2, e3])

As with a batch get, a batch put performs a single call to the service. The total size of the call is subject to the API call limits for the datastore. The entity count is also subject to a limit.

In Java, you can save entities using the put() method of a DatastoreService instance. As with get(), the method takes a single Entity for a single put, or an Iterable<Entity> for a batch put.

When the call to put() returns, the datastore is up to date, and all future queries in the current request handler and other handlers will see the new data. The specifics of how the datastore gets updated are discussed in detail in Chapter 6.

Deleting Entities

Deleting entities works similarly to putting entities. In Python, you can call the delete() method on the entity object, or you can pass entity objects or Key objects to the delete() function.

e = db.get('Entity', 'alphabeta')

db.delete([e1, e2, e3])

# Deleting without first fetching the entity:
k = db.Key('Entity', 'alphabeta')

In Java, you call the delete() method of the DatastoreService with either a single Key or an Iterable<Key>.

As with gets and puts, a delete of multiple entities occurs in a single batch call to the service, and is faster than making multiple service calls.

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