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Programming Google App Engine by Dan Sanderson

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Property Values

Each value data type supported by the datastore is represented by a primitive type in the language for the runtime or a class provided by the API. The data types and their language-specific equivalents are listed in Table 4-1. In this table, db is the Python package google.appengine.ext.db, and datastore is the Java package com.google.appengine.api.datastore.

Table 4-1. Datastore property value types and equivalent language types

Data typePython typeJava type
Unicode text string (up to 500 bytes, indexed)unicode or str (converted to unicode as ASCII)java.lang.String
Long Unicode text string (not indexed)db.Textdatastore.Text
Short byte string (up to 500 bytes, indexed)db.ByteStringdatastore.ShortBlob
Long byte string (not indexed)db.Blobdatastore.Blob
Booleanboolboolean
Integer (64-bit)int or long (converted to 64-bit long)byte, short, int, or long (converted to long)
Float (double precision)floatfloat or double (converted to double)
Date-timedatetime.datetimejava.util.Date
Null valueNonenull
Entity keydb.Keydatastore.Key
A Google accountusers.User...api.users.User
A category (GD)db.Categorydatastore.Category
A URL (GD)db.Linkdatastore.Link
An email address (GD)db.Emaildatastore.Email
A geographical point (GD)db.GeoPtdatastore.GeoPt
An instant messaging handle (GD)db.IMdatastore.IMHandle
A phone number (GD)db.PhoneNumberdatastore.PhoneNumber
A postal address (GD)db.PostalAddressdatastore.PostalAddress
A user rating (GD)db.Ratingdatastore.Rating

The datastore types in this table labeled “(GD)” are types borrowed from the Google Data protocol. These are supported as distinct native data types in the datastore, though most of them are implemented as text strings. Notable exceptions are GeoPt, which is a pair of floating-point values for latitude (–90 to +90) and longitude (–180 to +180), and Rating, which is an integer between 1 and 100.

Example 4-2 demonstrates the use of several of these data types in Python.

Example 4-2. Python code to set property values of various types

from google.appengine.ext import webapp
from google.appengine.ext import db
from google.appengine.api import users
import datetime

class Comment(db.Expando):
    pass

class CommentHandler(webapp.RequestHandler):
    def post(self):
        c = Comment()
        c.commenter = users.get_current_user()  # returns a users.User object
        c.message = db.Text(self.request.get('message'))
        c.date = datetime.datetime.now()
        c.put()

        # Display the result page...

Tip

When you use Python’s db.Expando or Java’s low-level datastore API, types that are widened to other types when stored come back as the wider datastore types when you retrieve the entity. For instance, a Java Integer comes back as a Long. If you use these APIs in your app, it’s best to use the native datastore types, so the value types stay consistent.

The data modeling interfaces offer a way to store values in these alternate types and convert them back automatically when retrieving the entity. See Chapters 7 and 8.

Strings, Text, and Blobs

The datastore has two distinct data types for storing strings of text: short strings and long strings. Short strings are indexed; that is, they can be the subject of queries, such as a search for every Person entity with a given value for a last_name property. Short string values must be less than 500 bytes in length. Long strings can be longer than 500 bytes, but are not indexed.

Text strings, short and long, are strings of characters from the Unicode character set. Internally, the datastore stores Unicode strings using the UTF-8 encoding, which represents some characters using multiple bytes. This means that the 500-byte limit for short strings is not necessarily the same as 500 Unicode characters. The actual limit on the number of characters depends on which characters are in the string.

The Python API distinguishes between short strings and long strings using Python data types. The Python built-in types unicode and str represent short string values. str values are assumed to be text encoded as ASCII, and are treated as UTF-8 (which is equivalent to ASCII for the first 128 characters in the character set). For long strings, the Python API includes a db.Text class, which takes a unicode or str value as an argument for its constructor.

# Short strings.
e.prop = "a short string, as an ASCII str"
e.prop = unicode("a short string, as a unicode value")

# A long string.
e.prop = db.Text("a long string, can be longer than 500 bytes")

The Java API makes a similar distinction, treating String values as short strings, and using the datastore.Text class to represent long text strings.

The datastore also supports two additional classes for strings of bytes, or “blobs.” Blobs are not assumed to be of any particular format, and their bytes are preserved. This makes them good for nontext data, such as images, movies, or other media. As with text strings, the blob types come in indexed and nonindexed varieties. The Python API provides the db.Blob class to represent blob values, which takes a str value as an argument for its constructor.

# A blob.  self.request.body is the body of the request in a
# webapp request handler, such as an uploaded file.
e.prop = db.Blob(self.request.body)

In Java, the blob types are datastore.ShortBlob and datastore.Blob.

Unset Versus the Null Value

One possible value of a property is the null value. In Python, the null value is represented by the Python built-in value None. In Java, this value is null.

A property with the null value is not the same as an unset property. Consider the following Python code:

class Entity(db.Expando):
    pass

a = Entity()
a.prop1 = 'abc'
a.prop2 = None
a.put()

b = Entity()
b.prop1 = 'def'
b.put()

This creates two entities of the kind Entity. Both entities have a property named prop1. The first entity has a property named prop2; the second does not.

Of course, an unset property can be set later:

b.prop2 = 123
b.put()

# b now has a property named "prop2."

Similarly, a set property can be made unset. In the Python API, you delete the property by deleting the attribute from the object, using the del keyword:

del b.prop2
b.put()

# b no longer has a property named "prop2."

In Java, the low-level datastore API’s Entity class has methods to set properties (setProperty()) and unset properties (removeProperty()).

Multivalued Properties

As we mentioned earlier, a property can have multiple values. We’ll discuss the more substantial aspects of multivalued properties when we talk about queries and data modeling. But for now, it’s worth a brief mention.

A property can have one or more values. A property cannot have zero values; a property without a value is simply unset. Each value for a property can be of a different type, and can be the null value.

The datastore preserves the order of values as they are assigned. The Python API returns the values in the same order as they were set.

In Python, a property with multiple values is represented as a single Python list value:

e.prop = [1, 2, 'a', None, 'b']

Note

Because a property must have at least one value, it is an error to assign an empty list ([] in Python) to a property on an entity whose Python class is based on the Expando class.

class Entity(db.Expando):
    pass

e = Entity()
e.prop = []  # ERROR

In contrast, the Model base class includes a feature that automatically translates between the empty list value and “no property set.” You’ll see this feature in Chapter 7.

In the Java low-level datastore API, you can store multiple values for a property using a Collection type. The low-level API returns the values as a java.util.List. The items are stored in the order provided by the Collection type’s iterator. For many types, such as SortedSet or TreeSet, this order is deterministic. For others, such as HashSet, it is not. If the app needs the original data structure, it must convert the List returned by the datastore to the appropriate type.

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