Michael Bayer wrote:
> but what happens if i say:
>
> q.select(or_(User.c.orders.items.item_name == 'item#4',
> User.c.orders.items.item_name == 'item #5'))
>
> if we naively convert c.orders.items.item_name=='item #4' into
> "user.user_id=orders.user_id and orders.order_id=items.order_id and
> items.item_name='item #4", then the other clause for item #5
> generates into the same thing and you get an inefficient query. i
> wonder also if some expressions above dont work correctly if the join
> conditions are repeated like that.
Can you give an example of the SQL (including the joins) that would be
generated by your statement above?
>
> its still better to say:
>
> q.select(or_(Item.c.item_name == 'item#4', Item.c.item_name == 'item
> #5'), from_obj=[c.orders.items])
>
> isnt it ? (User.c.orders.items would be a synonym for query.join_via
> ('orders', 'items'))
>
Right. It should be possible (although I'm not sure how simple) to combine
conditions using the rules for combining logical expressions (i.e. commutative,
transitive, etc.). For example:
(A == B AND C == D) OR (A == B AND C == E)
can be reduced to
(A == B) AND (C == D OR C == E)
So what we need is a way to take a group of expressions and reduce them to
their simplest form before generating the SQL. However, don't most main-stream
databases do this kind of optimization anyway? MySQL does
(http://dev.mysql.com/doc/refman/5.0/en/where-optimizations.html).
Having said all that, it may be simpler to use join_to/join_via in some cases
and maybe that's a good reason to keep those functions around. However, I think
this new syntax would still be very valuable in many cases.
FWIW, an equivalent but slightly more concise version of your query above would
be this:
q.select(user.c.orders.items.item_name.in_("item#4", "item #5"))
~ Daniel
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