Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  Luis A. Arce |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  QuerySet, filter,| Triage Stage:  Accepted
  slice  |
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Дилян Палаузов):

 * cc: Дилян Палаузов (added)


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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  Luis A. Arce |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  QuerySet, filter,| Triage Stage:  Accepted
  slice  |
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Дилян Палаузов):

 Is this fixed?  The last comments suggest so, but the status is "new".

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-

Comment (by Tim Graham ):

 In [changeset:"614dd44d0d2e5f8f12256835c3453f420f54c3b4"]:
 {{{
 #!CommitTicketReference repository=""
 revision="614dd44d0d2e5f8f12256835c3453f420f54c3b4"
 [1.6.x] Fixed #23817 -- Updated docs on QuerySet evaluation

 Removed inaccurate info about partial evaluation after refs #18702.
 Added information on modifying sliced QuerySets; refs #22503.

 Backport of 327df551e89a505c5756becee97c40198f38aff2 from master
 }}}

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-

Comment (by Tim Graham ):

 In [changeset:"593353d8af5b4b8d1a1e627712fe68ed593961d0"]:
 {{{
 #!CommitTicketReference repository=""
 revision="593353d8af5b4b8d1a1e627712fe68ed593961d0"
 [1.7.x] Fixed #23817 -- Updated docs on QuerySet evaluation

 Removed inaccurate info about partial evaluation after refs #18702.
 Added information on modifying sliced QuerySets; refs #22503.

 Backport of 327df551e89a505c5756becee97c40198f38aff2 from master
 }}}

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Ticket URL: 
Django 
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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-

Comment (by Tim Graham ):

 In [changeset:"327df551e89a505c5756becee97c40198f38aff2"]:
 {{{
 #!CommitTicketReference repository=""
 revision="327df551e89a505c5756becee97c40198f38aff2"
 Fixed #23817 -- Updated docs on QuerySet evaluation

 Removed inaccurate info about partial evaluation after refs #18702.
 Added information on modifying sliced QuerySets; refs #22503.
 }}}

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Ticket URL: 
Django 
The Web framework for perfectionists with deadlines.

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-

Comment (by tchaumeny):

 See also https://groups.google.com/forum/#!topic/django-
 developers/ELqU2xt_Qo0 (sliced querysets not usable in a
 `ModelChoiceField` because not filterable).

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-

Comment (by jorgecarleitao):

 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.

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:  QuerySet, filter,|  Needs documentation:  0
  slice  |  Patch needs improvement:  0
Has patch:  0|UI/UX:  0
  Needs tests:  0|
Easy pickings:  0|
-+-
Changes (by timo):

 * stage:  Unreviewed => Accepted


Comment:

 If we don't do this, let's document why not.

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Re: [Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
-+-
 Reporter:  ajaest   |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  1.6
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:
 Keywords:  QuerySet, filter,|  Unreviewed
  slice  |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by timo):

 * needs_better_patch:   => 0
 * type:  Bug => New feature
 * needs_tests:   => 0
 * needs_docs:   => 0


Comment:

 There was a recent [https://groups.google.com/d/topic/django-
 developers/oqgtvFE5y84/discussion django-dev thread] about this. I have
 not taken time to fully read it and determine if any consensus was
 reached.

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[Django] #22503: Inconsistent behavior when a QuerySet is sliced

[Django]

#22503: Inconsistent behavior when a QuerySet is sliced
--+
 Reporter:  ajaest|  Owner:  nobody
 Type:  Bug   | Status:  new
Component:  Database layer (models, ORM)  |Version:  1.6
 Severity:  Normal|   Keywords:  QuerySet,
 Triage Stage:  Unreviewed|  filter, slice
Easy pickings:  0 |  Has patch:  0
  |  UI/UX:  0
--+
 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 of `QuerySet`, something like `InmutableQuerySet` 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`.

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