On Wednesday, 1 February 2012 08:34:14 UTC, akaariai wrote:
>
> On Feb 1, 9:36 am, newme <dlli...@gmail.com> wrote:
> > do you mean that queryset will query database every time i call
> > user[0]?
>
> Yes. That is exactly what happens:
> In [7]: qs[0]
> Out[7]: <OrganisaatioOsa: THL - Terveyden ja hyvinvoinnin laitos>
> In [9]: print connection.queries
> [{'time': '0.011', 'sql': 'SELECT ... FROM "organisaatio_osa" LIMIT
> 1'}]
>
> In [10]: qs[0]
> Out[10]: <OrganisaatioOsa: THL - Terveyden ja hyvinvoinnin laitos>
> In [11]: print connection.queries
> [{'time': '0.011', 'sql': 'SELECT ... FROM "organisaatio_osa" LIMIT
> 1'},
> {'time': '0.001', 'sql': 'SELECT ... FROM "organisaatio_osa" LIMIT
> 1'}]
>
> If you do not want this to happen, you can evaluate your queryset into
> a list first by:
> objlist = list(qs[0:wanted_limit])
> and now objlist is just a regular Python list.
>
> The lazy evaluation of querysets can be a little surprising sometimes.
> Using django-debug-toolbar or just settings.DEBUG = True, and then
> print connection.queries is recommended :)
>
> - Anssi
Slight clarification: the slicing will only trigger a db call if the
queryset has not been evaluated. Converting it to a list is one way of
doing that, but if you iterate the queryset in any way it will populate the
internal cache and not call the database on subsequent slices.
--
DR.
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