Hi,
thanks for your answer!
>>>Now is there a way to force select_related tables any way in a count?
>
>select_related() is an optimization for data retrieval. It allows you
>to expand a select row to include related objects in a single query.
>
>It only follows the "1" side of "1-N" relations. That is, if a book
>has a foreign key on author, Book has a single Author. If you request
>N books, you will need to issue N+1 SQL queries. With select_related,
>this becomes 1 SQL query, and each row in the result set includes the
>details of the book and the details of the single related author.
Yes, this is the way and intention, I use related, too.
>This *does not* affect the row count. The number of books doesn't
>change as a result of including related authors in the result set.
>Hence, select_related() clauses are removed prior to a call to
>count().
But if you have a publisher foreign key on books, too, and you want to know the
amount of books one author has published at one publisher,
would I not do:
data = author.objects.select_related('books',
'books__publisher').filter(name__exact='Django Reihnard',
books__publisher__name__exact='music express')
amount = data.count()
if now a have a 4th table locations with addresses of the publisher, I would
do:
data = author.objects.select_related('books', 'books__publisher',
'books__publisher__locations').filter(name__exact='Django Reihnard',
books__publisher__name__exact='music express')
amount = data.count()
For just getting the count, the join to locations is omitted, as the number of
rows stays the same without locations, as there is no filter on tlocations
(publisher has a foreign key on locations).
If I expand the filter to:
books__publisher__locations__country__exact='germany', the join is done on a
count, too
(I use the variable data, as I first do a count on a query, afterwards I do
retrieve a *chunk* of data from the result set, so I do need to get the whole
amount of rows via the count())
Is this the wrong way to do it? Or is there / should there be a way to force
the missing join, if I need it for an extra?
In my "extra" I wanted to filter the locations in the query above based on a
radius search around a given longitude/latitude point, which can not be done
using orm. As I need the locations fields
on data retrieval, I thought I had to put locations into related
--
Hinnack
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