Question

I start with SQLAlchemy and i try to get my last insert (a Tweet) but i only get the sql query :

conf = readConf("../utils/conf.yaml")
schema = conf['bdd']['type'] + '://' + conf['bdd']['user'] + ':' + conf['bdd']['password'] + '@' + conf['bdd']['host'] + '/' + conf['bdd']['db'] + '?charset=utf8'

engine = create_engine(schema, echo=True)

Base = getBase()
Base.metadata.create_all(engine) 

Session = sessionmaker(bind=engine)
session = Session()
tq = session.query(Tweet).group_by(Tweet.tweet_id_uniq).filter(func.max(Tweet.tweet_id_uniq) == Tweet.tweet_id_uniq)
# tweet_id_uniq is a BIGINT autoincrement, so the highest value is the last Tweet.
print tq

The print result is the SQL query (wrapped for legibility):

SELECT tweets.tweet_id AS tweets_tweet_id, 
       tweets.tweet_id_uniq AS tweets_tweet_id_uniq, 
       tweets.user_id AS tweets_user_id, 
       tweets.user_id_uniq AS tweets_user_id_uniq, 
       tweets.tweet_text AS tweets_tweet_text, 
       tweets.created_at AS tweets_created_at, 
       tweets.in_reply_to AS tweets_in_reply_to, 
       tweets.geo_lat AS tweets_geo_lat, 
       tweets.geo_long AS tweets_geo_long, 
       tweets.screen_name AS tweets_screen_name, 
       tweets.name AS tweets_name, 
       tweets.profile_image_url AS tweets_profile_image_url, 
       tweets.source AS tweets_source 
FROM   tweets 
WHERE  max(tweets.tweet_id_uniq) = tweets.tweet_id_uniq 
GROUP BY tweets.tweet_id_uniq 

Why don’t I retrieve a Tweet ?

EDIT : If I add .one() or .fisrt() or .all(), i get this error :

2012-01-31 16:05:37,644 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2012-01-31 16:05:37,645 INFO sqlalchemy.engine.base.Engine SELECT tweets.tweet_id AS tweets_tweet_id, tweets.tweet_id_uniq AS tweets_tweet_id_uniq, tweets.user_id AS tweets_user_id, tweets.user_id_uniq AS tweets_user_id_uniq, tweets.tweet_text AS tweets_tweet_text, tweets.created_at AS tweets_created_at, tweets.in_reply_to AS tweets_in_reply_to, tweets.geo_lat AS tweets_geo_lat, tweets.geo_long AS tweets_geo_long, tweets.screen_name AS tweets_screen_name, tweets.name AS tweets_name, tweets.profile_image_url AS tweets_profile_image_url, tweets.source AS tweets_source 
FROM tweets 
WHERE max(tweets.tweet_id_uniq) = tweets.tweet_id_uniq
2012-01-31 16:05:37,645 INFO sqlalchemy.engine.base.Engine ()
...
File "/usr/local/lib/python2.6/dist-packages/SQLAlchemy-0.7.3-py2.6-linux-i686.egg/sqlalchemy/engine/default.py", line 330, in do_execute
cursor.execute(statement, parameters)
File "/usr/lib/pymodules/python2.6/MySQLdb/cursors.py", line 166, in execute
self.errorhandler(self, exc, value)
File "/usr/lib/pymodules/python2.6/MySQLdb/connections.py", line 35, in defaulterrorhandler
raise errorclass, errorvalue
sqlalchemy.exc.ProgrammingError: (ProgrammingError) (1111, 'Invalid use of group function') 'SELECT tweets.tweet_id AS tweets_tweet_id, tweets.tweet_id_uniq AS tweets_tweet_id_uniq, tweets.user_id AS tweets_user_id, tweets.user_id_uniq AS tweets_user_id_uniq, tweets.tweet_text AS tweets_tweet_text, tweets.created_at AS tweets_created_at, tweets.in_reply_to AS tweets_in_reply_to, tweets.geo_lat AS tweets_geo_lat, tweets.geo_long AS tweets_geo_long, tweets.screen_name AS tweets_screen_name, tweets.name AS tweets_name, tweets.profile_image_url AS tweets_profile_image_url, tweets.source AS tweets_source \nFROM tweets \nWHERE max(tweets.tweet_id_uniq) = tweets.tweet_id_uniq GROUP BY tweets.tweet_id_uniq' ()

Answer 1

You are trying to match on the greatest value and then group it. That value is unique. So you might as well just order by and get one.

tq = session.query(Tweet).order_by(desc(Tweet.tweet_id_uniq)).first()

tq is now your Tweet object with the largest id.

Incidentally, the problem is that your resulting query is incorrect sql. You can’t match a max value against a column from that same table without re-including the table (which probably isn’t what you want to do) or using a subquery (also awkward).