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Comment by Tycho | Hacker News Reader

GAE confuses me. How does one create a dynamic user interface? Does Django cover that? I'm trying to build a simple program that's a sort of GUI/wizard for composing SQL queries. It would be dead easy in .NET but I don't know where to start with GAE/Python. Is the idea that you build the front end in Javascript/HTML and your Python code does the processing behind the scenes? This is probably a n00b question but I'm finding it hard to get answers.

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You can compose queries like this:

query = myModel.all().filter("property_1 =", value)

query = query.filter("property_2 =", value)

Here's a gotcha... be sure to have a space between "property_1" and "=", "property_1=" won't work (but it won't raise an error either).

Be aware that the Datastore has very limited support for querying in general, and you will certainly not have anywhere near the kind of power you are used to when composing queries in .Net with something like LINQ.

GAE is a good start if you want to learn Python, but it is a terrible platform for web development. You will find yourself stymied and frustrated by its inflexibility, the need to give up your normalized data model from Day 1, lack of referential integrity, and the awful performance compared to a real SQL database.

Oh, and the constant outages, the scheduled maintenance that borks the whole system for a week, the backup tool that takes 4 hours to run and then crashes trying to restore the backups... I have a long list :)

Thanks for the advice. My program wont be doing any heavy lifting, it's just a simple tool to guide the SQL-writing process and save the user typing everything out down to the last semi-colon; a visual aid really. All I need is a tiny bit of graphical whizzbang, so it's not a totally static form - the labels/menus change somewhat, more fields/labels are added, according to the user input.

try to stop thinking in terms of SQL, if you can - although they call it GQL it's only superficially similar. One particularly big gotcha is that you can only return keys or whole objects in your queries, and if you return an object it has to deserialize all the data associated with that object, so think twice before storing binary data against a frequently queried object - look to the BlobStore (this is NOT the same as a BlobProperty) or a separate object to hold larger chunks of data.

This is something to consider at the get go, you'd be surprised how quickly performance degrades (ie a hundred or more items, not thousands)

I haven't used GAE, but if I generalize your question a little bit then I can answer it in the context of Django. You run a WSGI server that receives HTTP requests. It uses the regex-based URL mapping you write in your Django app to work out which view function to call (and, optionally, with which parameters). This function talks to the database, runs whatever logic you need, collects the relevant data for the UI and uses it to render a template, the result of which is then returned to the user.

Django templates look a lot like HTML, but with special Django stuff in them. You can get the general idea from this page: http://docs.djangoproject.com/en/dev/ref/templates/builtins/

And yes, you write in JS pretty much the same stuff that you'd always write in JS.

If you use web2py, web2py does that for example here is complete code to create a page that lets user post messages and read previous messages.

    m=db.define_table('message',Field('body'))
    def index():
        return dict(form=crud.create(m),rows=db(m).select())

It runs on GAE as it is (requires the scaffolding app).

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