Opened 11 years ago
Last modified 7 years ago
#22503 new New feature
Inconsistent behavior when a QuerySet is sliced
Reported by: | Luis A. Arce | Owned by: | nobody |
---|---|---|---|
Component: | Database layer (models, ORM) | Version: | 1.6 |
Severity: | Normal | Keywords: | QuerySet, filter, slice |
Cc: | Дилян Палаузов | Triage Stage: | Accepted |
Has patch: | no | Needs documentation: | no |
Needs tests: | no | Patch needs improvement: | no |
Easy pickings: | no | UI/UX: | no |
Description
I think that the current behavior of the QuerySet.slice method is not consistent. Just an example:
def limited_filtering(max_results=10, *args, **kwargs){ """ Returns a Queryset """ returns SomeModel.filter(*args, **kwargs)[:max_results] } # ... def other_method_somewhere(queryset){ """ Expects a Queryset of SomeModel and returns the same Queryset filtered again """ return queryset.filter(some_other_criteria) } other_method_somewhere(limited_filtering(*args, **kwargs)); # raises "AssertionError: Cannot filter a query once a slice has been taken."
Given the previous code, it is legit and fair that other_method_somewhere
expects, as in most situations, a filtrable QuerySet
. Not allowing a sliced QuerySet
to be filtered after being sliced adds an edge case difficult to handle when methods using a QuerySet
type are somehow semantically far away from each other (and thus, they don't know if the other is returning an sliced QuerySet).
Without knowing the exact reasons why this was implemented this way, the possible solutions I can come with are:
- The most natural one, to make the sliced
QuerySet
s filtrable. - To make all the returned query sets in which
can_filter == False
to return a new type ofQuerySet
, something likeInmutableQuerySet
in order to take advantage of other methods related to them (values_list
, etc.). - To make
QuerySet
's slices return list or other type of collection (I guess this is impossible without breaking backwards compatibility).
Actually, the last case is supposedly the way one would expect it to work according to the documentation:
https://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated
Slicing. As explained in Limiting QuerySets, a QuerySet can be sliced, using Python’s array-slicing syntax. Slicing an unevaluated QuerySet usually returns another unevaluated QuerySet, but Django will execute the database query if you use the “step” parameter of slice syntax, and will return a list. Slicing a QuerySet that has been evaluated (partially or fully) also returns a list.
By the way, there is also no reference in the documentation(or at least I could not find it) to the public method QuerySet.can_filter
.
Change History (9)
comment:1 by , 11 years ago
Type: | Bug → New feature |
---|
comment:2 by , 11 years ago
Triage Stage: | Unreviewed → Accepted |
---|
If we don't do this, let's document why not.
comment:3 by , 10 years ago
I will map the discussion in the thread to here: I believe the consensus was that this cannot be done:
Assume other_method_somewhere
uses a filter. The sequence would be
queryset.filter(first_clause)[:10].filter(second_clause)
Most people would assume (as pointed out in the thread) that the second clause would be applied to the last 10 entries, when it is not. Doing that is not possible in SQL without
subqueries and thus we explicitly forbid it.
comment:4 by , 10 years ago
See also https://groups.google.com/forum/#!topic/django-developers/ELqU2xt_Qo0 (sliced querysets not usable in a ModelChoiceField
because not filterable).
comment:9 by , 7 years ago
Cc: | added |
---|
There was a recent django-dev thread about this. I have not taken time to fully read it and determine if any consensus was reached.