Michael -- I see your point on the ORM creating objects that are
invariant on the primary keys and would use the combination of uid and
timestamp as the pk.
What I can't seem to figure out is how to add additional constraints to
the Mapper.
How would you define a mapper to always return the object defined by
the combination of uid and max(timestamp)?
When I define a second mapper, I get the following:
from sqlalchemy import *
metadata = BoundMetaData('sqlite://', echo=True)
users = Table('users', metadata,
Column('user_id', Integer, primary_key = True),
Column('timestamp', Integer, primary_key = True),
Column('name', String(80))
)
metadata.create_all()
ts = 100.
users.insert().execute(
dict(user_id = 1, name = 'Tom', timestamp = ts),
dict(user_id = 2, name = 'Dick', timestamp = ts),
dict(user_id = 3, name = 'Harry', timestamp = ts),
dict(user_id = 1, name = 'Tommy', timestamp = ts-1),
dict(user_id = 1, name = 'Tomi', timestamp = ts-2),
dict(user_id = 1, name = 'Tom', timestamp = ts-3),
)
class User(object) :
pass
user_map = mapper(User, users)
s = select([users.c.user_id,
func.max(users.c.timestamp).label('timestamp'), users.c.name])
user1_map = mapper(User, s.alias('t'), non_primary = True)
session = create_session()
q = session.query(user1_map)
q.select(users.c.user_id == 1)
[2006-08-07 11:13:25,845] [engine]: SELECT t.timestamp AS t_timestamp,
t.user_id AS t_user_id, t.name AS t_name
FROM users, (SELECT users.user_id AS user_id, max(users.timestamp) AS
timestamp, users.name AS name) AS t
WHERE users.user_id = ? ORDER BY t.oid
[2006-08-07 11:13:25,878] [engine]: [1]
[2006-08-07 11:13:25,880] [engine]: ROLLBACK
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "build/bdist.macosx-10.3-fat/egg/sqlalchemy/orm/query.py", line
226, in select
File "build/bdist.macosx-10.3-fat/egg/sqlalchemy/orm/query.py", line
232, in select_whereclause
File "build/bdist.macosx-10.3-fat/egg/sqlalchemy/orm/query.py", line
302, in _select_statement
File "build/bdist.macosx-10.3-fat/egg/sqlalchemy/orm/query.py", line
264, in instances
File "build/bdist.macosx-10.3-fat/egg/sqlalchemy/orm/session.py",
line 114, in execute
File "/Users/noon/Developer/test/base.py", line 244, in execute
File "/Users/noon/Developer/test/base.py", line 268, in
execute_clauseelement
File "/Users/noon/Developer/test/base.py", line 284, in
execute_compiled
File "/Users/noon/Developer/test/base.py", line 280, in proxy
File "/Users/noon/Developer/test/base.py", line 319, in _execute_raw
File "/Users/noon/Developer/test/base.py", line 338, in _execute
sqlalchemy.exceptions.SQLError: (OperationalError) no such column:
users.user_id 'SELECT t.timestamp AS t_timestamp, t.user_id AS
t_user_id, t.name AS t_name \nFROM users, (SELECT users.user_id AS
user_id, max(users.timestamp) AS timestamp, users.name AS name) AS t
\nWHERE users.user_id = ? ORDER BY t.oid' [1]
--Bill
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