Note: MRO was developed for an old version of web.py (0.2). I'm not maintaining it any more at this stage, but you're welcome to fork and improve it!

MRO is not an ORM. It’s not even the reverse of an ORM. Very simply, MRO maps rows to objects. It’s a thin layer on top of web.py’s equally thin database wrapper.

Why? Well, for a minimalist framework, I do like web.py’s close-to-the-SQL approach. But as soon as you have more than a couple of database operations, you find you’ve got code that repeats itself repeats itself. In the case of Gifty, I was repeating column names.

But I didn’t want a fancy ORM that gave me a new domain-specific language to worry about, or that supported every left, right, inner and outer join under the sun. I’ve found that for simple web apps, all I want is a bit of object with my row: a class that has attributes for columns, and lets you select and save rows.

I ended up with something that looks quite similar to Django’s database layer, albeit much simplified. (In hindsight, I may well have used Django for this project if I was doing it again.)

So here’s what MRO looks like:

# define a User object and its columns (SQL table name is "users")
class User(Table):
    _table = 'users'
    id = Serial(primary_key=True)
    username = String(secondary_key=True)
    hash = String()
    time = Timestamp(not_null=True, default='now()')

# create the users table with its columns and indexes
User.create()

# insert a new user into the database (defaults used for id and time)
bob = User(username='bob', hash='1234')
bob.save()

# fetch an existing user and update its hash column
bob = User('bob')
bob.hash = '4321'
bob.save()

# fetch an existing user (this time by primary key) and delete it
bob = User(42)
bob.delete()

# fetch an existing user (or None if no user called 'bill')
bill = User.get('bill')
if not bill:
    print 'Old Bill seems not to exist'

# get list of Users whose usernames start with 'ab' (also shows interpolation)
abusers = User.select(where='username LIKE $u', vars={'u': 'ab%'})

So, if you use web.py for a small web app, but you want a touch of class (ahem) with your database operations, go ahead and use MRO. Be aware that it’s an in-house tool (for instance, it only supports PostgreSQL at the moment).

See also my original blog entry about MRO.