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**requires version in trunk**

As you know web2py runs on almost any architecture and supports transparently many DB back-ends (SQLite, MySQL, PostgreSQL, Oracle).

Much of web2py runs on GAE too. Here we will try to clarify what runs, what does not run, and what to do about it.

The main issue with GAE is the absence of file system access. This means you cannot store session, tickets and uploaded files on the filesystem. They have to go in the databases.

GAE provides its own database based on BigTables. The Web2py ORM can access the GAE database and automatically generates GQL (Google Query Language) queries, but GQL has limitations compared with SQL and reflect in limitations in the types of ORM queries one can build.

## Example

Consider the following application that DOES NOT RUN on GAE:

### model db.py:

db=SQLDB("sqlite://db.db")

db.define_table('image',
      SQLField('title'),
      SQLField('filename','upload'))

db.image.title.requires=IS_NOT_EMPTY()

### controller default.py

def index():
        form=SQLFORM(db.image)
        if form.accepts(request.vars): response.flash='record inserted'
        images=db().select(db.image.ALL,orderby=db.image.title)
        return dict(form=form,images=images)

def edit():
        image=db(db.image.id==request.args[0]).select()[0]
        form=SQLFORM(db.image,image,upload=URL(r=request,f='download'))
        if form.accepts(request.vars): redirect(URL(r=request,f='index'))
        return dict(form=form)

def download():
        import os
        from gluon.contenttype import contenttype
        image=db(db.image.filename==request.args[0]).select()[0]
        response.headers['Content-Type']=contenttype(image.filename)
        return response.stream(os.path.join(request.folder,'uploads',image.filename)

### view default/index.html

{{extend 'layout.html'}}
    <h2>Post image</h2>
    {{=form}}

<h2>Images</h2>
    <ul>
    {{for image in images:}}
       <li>{{=image.title}}
       [<a href="{{=URL(r=request,f='edit',args=[image.id])}}">edit</a>]
       <br/>
       <img width="200px" src="{{=URL(r=request,f='download',args=[image.filename])}}"/>
       </li>
    {{pass}}
    </ul>

### view default/edit.html

{{extend 'layout.html'}}
    <h2>Edit image</h2>
    {{=form}}

It allows visitors to upload images with a title and to [edit] those title, images. All images are shown in the index page.

This app does not run because:

- uses SQLDB
- uses file based sessions
- uploads files in the filesystem

## Porting to GAE

In order to port it to GAE you need to change

db=SQLDB("sqlite://db.db")

into

try:
        from gluon.contrib.gql import *
        db=GQLDB()
    except:
        db=SQLDB("sqlite://db.db")
    session.connect(request,response,db=db)

The last line tells web2py to store sessions in DB. A `web2py_session` table is created automatically.

You need to change the model to

db.define_table('image',
       SQLField('title'),
       SQLField('filename','upload',uploadfield='data'),
       SQLField('data','blob',default=''))

i.e. for every field of type upload, add a new blob field where to upload the file into. The blob field must default to an empty string.

You also need to replace the last line of the download controller function from

return response.stream(os.path.join(request.folder,'uploads',image.filename)

return image[db.image.filename.uploadfield]

So that you retrieve files from the db instead of the filesystem.

That's it! Code written for GAE also works without GAE since it is valid web2py code.

## On Complex Queries

These queries will work on GAE

db(db.image.id==1).select()
    db(db.image.id==1).delete()
    db(db.image.id==1).update(title='title')

db(db.image.title<'z').select()
    db(db.image.title>'a').delete()
    db(db.image.title=='test').update(title='title')

db((db.image.title<'z')&(db.image.title>'a')).select(db.image.ALL,sortby=db.image.title,limitby=(0,10))

but on GAE you WILL NOT BE ABLE to:

- use OR (operator "|") in queries
- use the not operators ("!=" and "~")
- use LIKE (operator "like") and IN (operator "belongs")
- use groupby
- mix queries by id and queries by other fields like in `(db.image.id==id)&(db.image.title=='')`
- manipulate date, time, datetime
- select anything other than db.table.ALL
- do JOIN; i.e. build queries involving more than one table

## What else does not work

The following functionalities do not work:

- admin
- tickets (they are logged but not stored in files)

We are working to resolve some of these issues. 
Admin is available when doing local development.

Resources

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Development Trunk (svn)

Contact

Massimo Di Pierro
School of Computing
DePaul University
243 S. Wabash Ave
Chicago, IL 60604 (USA)
mdipierro@cs.depaul.edu

web2py AlterEgo

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