On Friday, March 2, 2012 7:24:00 PM UTC+2, Tom Evans wrote:
>
> Hi all
>
> I have a particular query that requires me to use a RawQuerySet - I
> need to left join the table to itself in order to select the highest
> 'priority' row from the table for each distinct value of a foreign key
> on the model, and I need to join to the related table in order to
> filter the results.
>
> Having generated the queryset, I then want to make a dictionary of {
> foreign_key_object : model_object }. So I have this working, but I
> cannot use select_related() with a RawQuerySet, and so this runs N+1
> queries, where N is the number of distinct foreign keys.
>
> Here is some code, which might explain it better:
>
> connection.queries=[]
> base_products_qs = Product.objects.raw(
> """
> SELECT idp_product.*, idp_productclass.*
> FROM idp_product
> JOIN idp_productclass
> ON idp_product.product_class_id = idp_productclass.id
> LEFT JOIN idp_product p2
> ON idp_product.product_class_id = p2.product_class_id
> AND p2.class_priority < idp_product.class_priority
> WHERE p2.id IS NULL and idp_productclass.product_type != 4
> """)
> base_products = dict([ (p.product_class, p) for p in base_products_qs ])
> len(connection.queries) # 7 queries (6 product classes)
>
> Is there any simple way around this? I can reduce it to two queries
> already, but it seems wrong to select out the info I want, throw it
> away, and then fetch it again.
>
Some suggestions:
- If the 2-query version is fast enough, use it. Premature optimization
and all that... :)
- You could of course manually iterate through the SQL and instantiate
the models by hand.
- Beware of the left join, it seems if you have a lot of rows with
different class_priorities for each product_class_id you might be in
trouble. The filtering on IS NULL happens after doing the join, so before
the filter you could have a lot of intermediate rows. It might be there is
no problem depending of the amount of rows and SQL vendor. Hard to know
without testing. There are a couple of other ways to do the query, either
by subquery doing a select min(class_priority), product_class_id or if you
are using PostgreSQL, distinct on would do the job for you, too.
All that being said, I would really like the ability to have select_related
available for RawQuerySet. It might be judged to be a feature not needed
commonly enough, so it is possible it would not get in even with a good
quality patch. However, if it had a nice API, and the implementation reused
the code of QuerySet select_related, I think it could have a chance to get
in.
- Anssi
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