Googe App Engine CodelabMarzia NiccolaiMay 28-29, 2008

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Step 1: Download the SDK

• You must have Python 2.5 installed on your computer to use the App Engine SDK

• http://code.google.com/p/googleappengineoWindows InstalleroMac App Engine LauncheroZip Archive

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Building a Wiki

• Today we are going to build a wiki with Google App Engine

• You can download the code from:• http://code.google.com/p/google-app-engine-

codelab

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Wiki Content

• Step 1 - Basic Wiki• Step 2 - Wiki that uses Markup and

displays the author• Step 3 - Wiki that supports revisions• Step 4 - Wiki that displays user page• Step 5 - Wiki that fetches user feeds

During the Codelab, we will be going over steps 1&2 and building step 3

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Wiki Data

• To start with, we will have wiki pages with a title & a text body

• With App Engine you store you data as objects called entities

• Entities have 1 or more properties of supported data types

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Data Types• IntegerProperty, FloatProperty, BooleanProperty• StringProperty, TextProperty• DateTimeProperty• ListProperty• ReferenceProperty, SelfReferenceProperty• UserProperty• BlobProperty• LinkProperty, EmailProperty• Other rich properties...• http://code.google.com/appengine/docs/datastore/

typesandpropertyclasses.html

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Defining a Model

• Define your data models as a Python class• To create an entity use the class

constructor• Call put() on the object to add it to the

datastore

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Defining the Wiki Modelfrom google.appengine.ext import db

class WikiPage(db.Model):  title = db.StringProperty()  body = db.TextProperty()

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Create, Update, Delete

•Once you have a data object (new or existing), calling put() writes that object to the datastore

object = WikiPage(title=my_title, body=my_body) object.put()

•To delete an object from the datastore, call delete() on the object

        # assume we have retrieved object        object.delete()

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Querying for Data

• Google provides two methods for querying data

• Today we’ll use GQL, a SQL-like query language

• We also have a query interface, which you can read more about at:

• http://code.google.com/appengine/docs

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GQL

• Google App Engine returns entire entities based on your queries

• You can filter the data based on equalities and inequalities

• You can order the data

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Sample GQL QueriesSELECT * FROM Person WHERE birth_year >= :min AND birth_year <= :max

SELECT * FROM Person WHERE birth_year >= :min ORDER BY birth_year, last_name

SELECT * FROM Person WHERE last_name = "Smith" AND height < 72 ORDER BY height DESC

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Codelab: Building our wiki

• Allow our users to:o View Wiki pageso Edit Wiki pages

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app.yaml - Describing our App

• The app.yaml specifies the application configuration for Google App Engine

• Specify the application name and version:

application: wikiversion: 1

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Wiki’s app.yaml

• Specify the application’s script handlers:

handlers:- url: /.* script: main.py

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app.yaml: complete

application: wikiversion: 1runtime: pythonapi_version: 1

handlers:- url: .* script: main.py

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WebApp Framework

• WSGI framework for handling requests• Uses Request Handlers classes to serve

pages• Returns entire output when handler exits

(no streaming)

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Wiki Page Handlers Requirements

• View Wiki Pages (/view/WikiTopic)o Request page /view/WikiTopico Directs you to the page on WikiTopic

• Add/Edit Wiki Pages (/edit/WikiTopic)o Create pages that don’t existo Edit existing pages

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Define our View Handler

• Redirect request from http://myapp.apppost.com/ to http://myapp.appspot.com/view/StartPage

• When you request http://myapp.appspot.com/view/WikiPageo Display Content, if it exists!o Allow user to Add or Edit the Page

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Define the Edit Handler

• When the user requests /edit/WikiPage:o Give them form with a text box to enter contento Post the form to /save/WikiPage

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Define the Save Handler

• When the user posts a request to /save/WikiPage:o Get the body of the requesto Create a data objecto Store the object to the datastoreo Redirect the user to /view/WikiPage

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Wiki: Take 1

• http://localhost:8080/

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Wiki: Take 1 Issues

• We do no processing on the form input, and do no formatting

• We are ignoring the Wiki author!

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Wiki Take 2: Processing Form input

• Use the ‘markdown’ third party library to allow formatting

• Find WikiWords (words that are camel cased) and replace them with links

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Wiki Take 2: Authors

• Let people log in and out of our website• Only allow people who are logged in to edit

pages• Store the author of the page

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Wiki Take 2 in Action

• http://localhost:8080

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Wiki Take 3 - Revisions

• You will be adding revision history• Each time someone edits the page,

associate that revision as the current revision of the wiki page

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Reference Properties

• Enable 1:many, many:many entity relationships

• Allows entity to store a reference to another entity

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Using ReferencePropertyclass Story(db.Model):  story_text = db.TextProperty()

class Comment(db.Model):  story = db.ReferenceProperty(Story)  comment_text = db.TextProperty()  user = db.UserProperty()

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Retrieve a Reference Propertya_comment = Comment.gql(‘WHERE user = :1’, the_user).get()

self.response.out.write(‘Our user commented on this story: %s’ % a_comment.story.story_text)

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Back Referencesa_story = Story.all().get()

for a_comment in a_story.comment_set():  self.response.out.write(‘by:%s<br />%s<br />’ %(a_comment.user.nickname, a_comment.comment_text) )

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ReferenceProperty & the Wiki

• New object model for wiki take 3class WikiUser(db.Model)• Store user information in an entityclass WikiContent(db.Model)• Creates the parent wiki pageclass WikiRevision(db.Model)• Stores each revision of the Wiki Page

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Wiki User

• In the future we want our wiki page to have user profiles so we can create a user entity that stores information about our user:

class WikiUser(db.Model):  wiki_user = db.UserProperty()  joined = db.DateTimeProperty(auto_now_add=True)  wiki_user_picture = db.BlobProperty()  user_feed = db.StringProperty()

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Wiki Page

class WikiContent(db.Model):  title = db.StringProperty()

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Wiki Revisionclass WikiRevision(db.Model):  wiki_page = db.ReferenceProperty(WikiContent)  revision_body = db.TextProperty(required=True)  author = db.ReferenceProperty(WikiUser)  created = db.DateTimeProperty(auto_now_add=True)  version_number = db.IntegerProperty()

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Now it’s your turn!

• Using the model defined above, add support for revisions to our wiki!

• We’ll be walking around to offer assistance as needed