#27936: ASCII Art for docs "Spanning multi-valued relationships"
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Reporter: Thomas Güttler | Owner: nobody
Type: Uncategorized | Status: new
Component: Documentation | Version: 1.10
Severity: Normal | Resolution:
Keywords: | Triage Stage: Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Comment (by Josh Smeaton):
I think there's definitely scope to improve the docs around multi valued
relationships, but I don't think ASCII art (or that diagram) is really the
right way of doing it. For some added confusion, the docs fail to mention
that duplicates are possible with the second query (with multiple filters)
if there is an entry with the headline Lennon AND it was posted in 2008.
It seems the docs go through a lot of effort to avoid mentioning that a
second filter causes the query to create a second join to the same table.
I'm not proposing new language for this myself, but I've seen many an
experienced developer get caught by this without understanding what was
actually happening with the underlying query. I'd very much like to see
these docs improved in some way.
Here's some shell output for those curious about what's happening:
{{{
In [1]: from datetime import date
In [2]: d2008 = date(2008, 6, 6)
In [3]: d2009 = date(2009, 6, 6)
In [4]: both = Blog.objects.create(name='Match Both')
In [5]: Entry.objects.create(blog=both, headline='1 Lennon 1',
body_text='body', pub_date=d2008)
Out[5]: <Entry: Entry object>
In [6]: Entry.objects.create(blog=both, headline='2 Lennon 2',
body_text='body', pub_date=d2009)
Out[6]: <Entry: Entry object>
In [7]: Entry.objects.create(blog=both, headline='3 Blah 3',
body_text='body', pub_date=d2008)
Out[7]: <Entry: Entry object>
In [8]: Blog.objects.filter(entry__headline__contains='Lennon',
entry__pub_date__year=2008)
Out[8]: <QuerySet [<Blog: Match Both>]>
In [9]: justdate = Blog.objects.create(name='Match Date Only')
In [10]: Entry.objects.create(blog=justdate, headline='4 Blah 4',
body_text='body', pub_date=d2008)
Out[10]: <Entry: Entry object>
In [11]: justheadline = Blog.objects.create(name='Match Headline Only')
In [12]: Entry.objects.create(blog=justheadline, headline='5 Lennon 5',
body_text='body', pub_date=d2009)
Out[12]: <Entry: Entry object>
In [13]: Blog.objects.filter(entry__headline__contains='Lennon',
entry__pub_date__year=2008)
Out[13]: <QuerySet [<Blog: Match Both>]>
In [14]:
Blog.objects.filter(entry__headline__contains='Lennon').filter(entry__pub_date__year=2008)
Out[14]: <QuerySet [<Blog: Match Both>, <Blog: Match Both>, <Blog: Match
Both>, <Blog: Match Both>]>
}}}
And the queries:
{{{
# Blog.objects.filter(entry__headline__contains='Lennon',
entry__pub_date__year=2008)
SELECT
"scratch_blog"."id",
"scratch_blog"."name",
"scratch_blog"."tagline"
FROM "scratch_blog"
INNER JOIN "scratch_entry" ON ("scratch_blog"."id" =
"scratch_entry"."blog_id")
WHERE (
"scratch_entry"."pub_date" BETWEEN '2008-01-01' :: DATE AND '2008-12-31'
:: DATE)
AND "scratch_entry"."headline" LIKE '%Lennon%'
);
#
Blog.objects.filter(entry__headline__contains='Lennon').filter(entry__pub_date__year=2008)
SELECT
"scratch_blog"."id",
"scratch_blog"."name",
"scratch_blog"."tagline"
FROM "scratch_blog"
INNER JOIN "scratch_entry" ON ("scratch_blog"."id" =
"scratch_entry"."blog_id")
INNER JOIN "scratch_entry" T3 ON ("scratch_blog"."id" = T3."blog_id")
WHERE (
"scratch_entry"."headline" LIKE '%Lennon%'
AND T3."pub_date" BETWEEN '2008-01-01'::date AND '2008-12-31'::Date)
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/27936#comment:1>
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