MRO: Map Rows to Objects with web.py
6 January 2010, by Ben 2 comments
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).
2 comments (oldest first)
Thanks, Alan. I’m not actually a github user, but you’re welcome to put it up there if you like.
This looks pretty nice and Python could certainly use another database option. Post it on github!