"and" type query over related objects

Hi,

I need to be able to query an object that has a set of
related  
objects - the query is to find which objects contain the
same set of  
query objects. For example the main object could be a
research paper  
and the related object would be a set of topics that the
papers could  
contain.

class Topic(models.Model):
     label = models.CharField(maxlength=255)

class Paper(models.Model):
	topic = models.ManyToManyField(Topic)

I have tried a few approaches, I thought I could simply
chain a  
series of filters or AND some Q objects:

In [28]: p=Paper.objects.filter(topic=t2).filter(topic=t)

In [29]: p
Out[29]: []


In [39]: Paper.objects.filter(Q(topic=t)|Q(topic=t2))
Out[39]: [<Paper: title1>, <Paper: title2>]

In [40]: Paper.objects.filter(Q(topic=t),Q(topic=t2))
Out[40]: []

But these don't work and I end up with an empty query set,
but there  
is  a Paper object with both a t and t2 object.

Thanks,
jms.

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> I need to be able to query an object that has a set of
related
>  objects - the query is to find which objects contain
the same
> set of query objects. For example the main object could
be a
> research paper and the related object would be a set of
topics
> that the papers could contain.


This is a popular question that I've found a solution to. 
It 
might be kinder in the queryset refactor (Malcolm?), but for
the 
time being, this is what I've used:

http://groups.google.com/group
/django-users/browse_thread/thread/24de9d4b74935296/

Because of the way the underlying SQL builds the query, you

basically end up asking for two contradictory conditions of
the 
same row:

   WHERE x = 1 AND x = 2

which, as you discovered, returns no rows.

The solution I've found is to use an extra() call to build
an 
IN/EXISTS[1] subselect that asks what I want.  Thus, it
becomes 
something like

   WHERE EXISTS(
     SELECT 0
     FROM <innertbl> i
     WHERE <condition1> AND
       i.<fk> = <outertbl>.id
     ) AND EXISTS(
     SELECT 0
     FROM <innertbl> i
     WHERE <condition2> AND
       i.<fk> = <outertbl>.id
     )

-tim

[1] FWIW, I've found EXISTS to be faster in PostgreSQL and
IN to 
be faster in SQLServer; haven't profiled MySQL or sqlite.






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