On 19 Nov, 01:33, Malcolm Tredinnick <[EMAIL PROTECTED]> wrote:
> On Tue, 2008-11-18 at 07:09 -0800, Nicola Murino wrote:
> > Hi all,
>
> > I read a lot of documentation about caching queryset but seems nothing
> > is useful for my purpose:
>
> > I have to load a treeview and so perform virtually infinite recursion,
> > I want to minimize database access, here is my simplified model
>
> > class Nodes(models.Model):
> > name = models.CharField(max_length=255)
> > sublivello_di = models.ForeignKey('self', blank=True, null=True,
> > related_name='sublivelli_set')
>
> > class ObjectType1(models.Model):
> > ...
> > ...
> > node=models.ForeignKey(Nodes)
>
> > class ObjectType2(models.Model):
> > ...
> > ...
> > node=models.ForeignKey(Nodes)
>
> > and so on.
>
> > I would like to select all the object before recursion, for example:
>
> > allnodes=Nodes.objects.select_related('sublivello_di').all()
> > allobject1=ObjectType1.objects.select_related().all()
>
> > however when I do some filtering django hits the database, however all
> > the objects are already fetched, I need only a subset of this.
>
> What do you mean "when you do some filtering"? Querysets only access the
> database when you access the individual results inside the queryset. So,
> for example, neither of the above two statements cause any database
> operations. It's only when you try to work with the data in allnodes or
> allobject1 that the database will be accessed.
>
> So if you're adding extra filters to those querysets, you can happily do
> so and it won't cause any database accesses (again, until you use the
> objects' data).
>
> If you could provide a short code fragment showing what you mean by
> filtering (of the query in the view? At the template level?), that might
> help.
>
> Also, how are you determining that the database is being accessed each
> time?
>
> Regards,
> Malcolm
Thanks for your answer,
I'm determining that the database is being accessed using postgres log
and the output of connection.queries
If i do :
no=Nodes.objects.all()
the queryset are lazy so at this point no query is performed, if I do
no.filter(sublivello_di=something) yet no query is performed.
Now to populate my tree I have a recursive function, I have to find
the childs for each node so I have a for on no objects and this cause
queryset evaluation and so db access, the I call a recursive function
to find the child for each zone
def recursive(n):
nodes=Nodes.objects.select_related().filter(sublivello_di=n)
o1=ObjectType1.objects.select_related().filter(node=n)
.....
for o in o1: #db access
.....
for subn in nodes: # db access
recursive(subn)
for n in no:
recursive(n)
I would like to do something similar to this:
def recursive(n,o1):
nodes=Nodes.objects.select_related().filter(sublivello_di=n)
o1=o1.filter(node=n)
.....
for o in o1: #no db access I passed the full queryset so why
access the db?
.....
for n in no:
# I want access the db here, so if I do o1.filter no db access is
required
o1=ObjectType1.objects.all()
recursive(n,o1,..)
I hope my explaination is clear enough, sorry for my english,
regards
Nicola
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