On Dec 11, 2011, at 11:55 AM, lestat wrote:
> /sqlalchemy/sql/expression.py:1869: SAWarning: The IN-predicate on
> "comment.reply_id" was invoked with an empty sequence. This results in
> a contradiction, which nonetheless can be expensive to evaluate.
> Consider alternative strategies for improved performance.
> return self._in_impl(operators.in_op, operators.notin_op, other)
>
> I try fix this warning.
>
> user_comments_ids = tuple(x[0] for x in
> getCommentsForUser(user_id).values('id'))
> if user_comments_ids:
> comments_replies =
> Comment.query.filter(Comment.reply_id.in_(user_comments_ids))
> else:
> # empty query
> comments_replies = Comment.query.filter_by(id=None)
> return comments_replies
>
> but exists better solution?
>
> Why not add to sqlalchemy Query method like .all(), .one(), new
> method .none() like in django orm?
>From what I'm reading, none() just means "return an empty list". SQLA's
>Query object is not generative beyond the point of all()/one(), it appears
>like Django's .none() produces a generative construct. So the SQLAlchemy
>analogue of .none() compared to .all() or one() would be "[]". As far as
>a Django-like analogue, where the Query object would produce a
>False-evaluating object which is then further usable in larger queries,
>SQLAlchemy would prefer you simply don't add "False" predicates to your query
>- it's a situation that can't be concealed without leading to inefficient or
>unnecessary queries.
In the above case, you'd just say "comments_replies = []".
My understanding is that Django resolves "x.in_([])" to an expression like
"false", or "1 != 1". This approach isn't correct in all cases, namely "NOT
(x IN ())" where "x" is NULL.
--
You received this message because you are subscribed to the Google Groups
"sqlalchemy" group.
To post to this group, send email to sqlalchemy@googlegroups.com.
To unsubscribe from this group, send email to
sqlalchemy+unsubscr...@googlegroups.com.
For more options, visit this group at
http://groups.google.com/group/sqlalchemy?hl=en.
