#10870: Aggregates with joins ignore extra filters provided by setup_joins
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Reporter: fas | Owner: fas
Status: new | Milestone:
Component: ORM aggregation | Version: 1.0
Keywords: orm, aggregation, join, contenttypes, filter | Stage:
Unreviewed
Has_patch: 0 |
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(In case the description that follows is not understandable, please ask me
for a better explanation).
An example where extra filters in joins are used are reverse generic
relations ([http://docs.djangoproject.com/en/dev/ref/contrib/contenttypes
/#reverse-generic-relations]).
{{{
class TaggedItem(models.Model):
content_type = models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
content_object = generic.GenericForeignKey('content_type',
'object_id')
class Bookmark(models.Model):
url = models.URLField()
tags = generic.GenericRelation(TaggedItem)
}}}
By querying {{{ Bookmark.objects.filter(tags=some_tag) }}} the generic
relation adds extra filters to ensure the correct content type. Those
extra filters however are ignored when using aggregates, which leads to
wrong results.
For example, the following query is not correct because it does not check
the content_type.
{{{
Bookmark.objects.aggregate(Count('tags'))
}}}
What this aggregate actually calculates per bookmark is not the amount of
tags for this bookmark but the amount of tags for any model referenced by
TaggedItem with the same object id.
The error is isolated in db/models/sql/query.py in the add_aggregate
method:
{{{
field, source, opts, join_list, last, _ = self.setup_joins(
field_list, opts, self.get_initial_alias(), False)
# Process the join chain to see if it can be trimmed
col, _, join_list = self.trim_joins(source, join_list, last,
False)
}}}
The last dummy element _ is normally named 'extra' in other uses of
setup_joins and contains the extra filters needed to make it work. Adding
{{{ self.add_filter(*extra) }}} after setup_joins fixes this problem.
--
Ticket URL: <http://code.djangoproject.com/ticket/10870>
Django <http://code.djangoproject.com/>
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