Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Gwildor):

 Just to add, it would be great if not only `.values()` would be supported,
 but also `.values_list(flat=True)`.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:
 |  skrilladeville
 Type:  New feature  |   Status:  assigned
Component:  Migrations   |  Version:  master
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values,| Triage Stage:  Accepted
  payroll, pay   |
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by skrilladeville):

 * keywords:  prefetch, values => prefetch, values, payroll, pay
 * owner:  nobody => skrilladeville
 * status:  new => assigned
 * component:  Database layer (models, ORM) => Migrations


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Dan LaManna):

 * cc: Dan LaManna (added)


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Micah Lyle):

 * cc: Micah Lyle (added)


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Micah Lyle):

 I took a stab at this during the DjangoCon 2019 sprints.

 I had very limited experience with the prefetching/ORM internals before
 this, so there could be workarounds/solutions that I'm unaware of, but
 here's what I found so far.

 One thing I tried to tackle is handling the many to many case with just
 `values()`. My goal was to take the `'_prefetch_related_val_%s'` in the
 `queryset.extra(...)` call in the `get_prefetch_queryset()`
 `ManyRelatedManager` and add/apply it to whatever `values()` call was made
 and then join that by doing `pop()` from the dictionary instead of a
 `getattr` call (which would effectively give the developer the same query
 that they originally made by modifying it under the hood). I ran into some
 issues doing this though:

 "Any `extra()` call made after a `values()` call will have its extra
 selected fields ignored" (from the Django 2.2 docs)."

 This means that the `queryset.extra(...)` call in the `ManyRelatedManager`
 (which from my quick search appears to be the only remaining usage of
 `extra(...)` across the entire Django codebase (except the tests)) is
 effectively ignored, and even if I try to add a `.values(*existing_values,
 '_prefetch_related_val_%s')` call, it returns this error (from my example
 test/use case using the models from the prefetch tests):

 {{{
 queryset.values('id', 'title', '_prefetch_related_val_author_id')

 django.core.exceptions.FieldError: Cannot resolve keyword
 '_prefetch_related_val_author_id' into field. Choices are:
 _prefetch_related_val_author_id, authors, bio, bookwithyear,
 first_time_authors, id, read_by, title
 }}}

 Is this a bug? It says it can't resolve a field and then says that field
 is a choice.

 I thought about trying to workaround it by essentially "undoing" the
 `values()` part of the `queryset` and then reapplying it at the end with
 the value from the join table (which then gets `pop`ed, but I'm not sure
 if that's feasible/doable and what that would actually look like. Besides,
 this entire solution so far feels somewhat hacky in the first place.

 I think that whatever change makes this happen may require some sort of
 specification on the developer's end for how to apply the join from the
 prefetched dictionaries to the existing list of instances. Maybe even
 writing parts of a custom prefetcher and specifying that custom prefetcher
 as a fourth keyword argument (something I explored, but didn't get very
 far) to the `Prefetch` object, and then tweaking the logic in
 `prefetch_related_objects` and other functions to support using that
 fourth keyword argument.

 I'm also wondering if there's a way with Django 3.1+ technology/API to
 rewrite the `queryset.extra(...)` call in `ManyRelatedManager`. An out
 there idea I had was replacing the `queryset.extra(...)` call with a query
 on the through table and then doing a select related to the foreign key
 that is on the side we want to bring in to join to the existing instances
 (in the case above, we'd grab
 
`Book.authors.through.objects.filter_based_on_what_we_have_for_books_and_authors().select_related('book')`
 and then transform to a values result from there but that would (I think)
 prevent aggregations in the first place and run into a number of other
 problems.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Alexander Schepanovski):

 Anyone on this? Would be great to have.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Simon Charette):

 No one has claimed it yet Alexander, feel free to propose a patch!

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Aymeric Augustin):

 Just came across the same need. That would be a nice feature.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by kimsia):

 What's the workaround for this if I have a similar need?

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Thomas Riccardi):

 * cc: Thomas Riccardi (added)


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:
 |  AlexeyNigin
 Type:  New feature  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by AlexeyNigin):

 * owner:  nobody => AlexeyNigin
 * status:  new => assigned


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  (none)
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by AlexeyNigin):

 * owner:  AlexeyNigin => (none)
 * status:  assigned => new


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by mlorant):

 Yes, same error & traceback using `PayrollLine.objects.values('code')`
 only.

 The error seems obvious: the prefetch mechanism tries to access to
 attributes for some reason, but it got a dict from the query, not model
 instances...

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by charettes):

 * stage:  Unreviewed => Accepted


Comment:

 I'm not sure if this should be considered a ''New Feature'' or a ''Bug''.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by akaariai):

 I'd say there is a bug and a new feature. We should error nicely on values
 queries, and of course, ability to prefetch values queries would be a nice
 feature for aggregations specifically.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by charettes):

 * has_patch:  0 => 1


Comment:

 Submitted a [https://github.com/django/django/pull/6545 PR] for the
 ''bug'' part. This should be converted to a feature request once it's
 merged.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Simon Charette ):

 In [changeset:"7ec330eeb96d0874949eacb8ed1bbb97e11807e1" 7ec330e]:
 {{{
 #!CommitTicketReference repository=""
 revision="7ec330eeb96d0874949eacb8ed1bbb97e11807e1"
 Refs #26565 -- Errored nicely when using Prefetch with a values()
 queryset.

 Thanks Maxime Lorant for the report and Anssi for the suggestion.
 }}}

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by charettes):

 * type:  Bug => New feature


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by charettes):

 * has_patch:  1 => 0


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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by Simon Charette):

 > What's the workaround for this if I have a similar need?

 Perform the prefetch queryset and attribute assignment yourself. In the
 end `prefetch_related` only does the following

 It transforms

 {{{#!python
 Authors.objects.filter(popular=True).prefetch_related(Prefetch("books",
 to_attr="prefetched_books"))
 }}}

 Into

 {{{#!python
 authors = Authors.objects.filter(popular=True)
 authors_books = defaultdict(list)
 for book in Book.objects.filter(author__in=authors):
 authors_books[book.author_id].append(book)
 for author in authors:
 setattr(author, "prefetched_books", authors_books[author.pk])
 }}}

 So as long as the back references is included in your `values` (which is
 something the `prefetch_related` code would need to do anyway and the part
 Micah ran into issues with) you should be able to achieve the same thing
 with two loops.

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Re: [Django] #26565: Allow Prefetch query to use .values()

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  Maxime Lorant|Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  dev
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:  Accepted
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by Ülgen Sarıkavak):

 * cc: Ülgen Sarıkavak (added)

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

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Re: [Django] #26565: Allow Prefetch query to use .values() (was: Allow Prefetch query to returns aggregations)

[Django]

#26565: Allow Prefetch query to use .values()
-+-
 Reporter:  mlorant  |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:
 Keywords:  prefetch, values | Triage Stage:
 |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by charettes):

 * needs_better_patch:   => 0
 * version:  1.9 => master
 * needs_tests:   => 0
 * needs_docs:   => 0


Comment:

 I doubt this is related to the linked PR or aggregation. I believe
 `Prefetch.queryset` cannot be used with `values()` queryset.

 Please confirm you get a similar crash with the following queryset:

 {{{#!python
 queryset = PayrollLine.objects.values('code')
 Order.objects.prefetch_related(
 Prefetch('pay_lines', queryset=queryset, to_attr='pay_lines_values')
 )
 }}}

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