Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  duplicate
 Keywords:   | Triage Stage:  Accepted
Has patch:  1|  Needs documentation:  1
  Needs tests:  1|  Patch needs improvement:  1
Easy pickings:  0|UI/UX:  0
-+-
Changes (by timgraham):

 * status:  new => closed
 * resolution:   => duplicate


Comment:

 Duplicate of #16211

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by aaugustin):

 * stage:  Design decision needed => Accepted


Comment:

 This feature shouldn't be expected to give meaningful results on anything
 other than a boolean value.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Design
 Keywords:   |  decision needed
Has patch:  1|  Needs documentation:  1
  Needs tests:  1|  Patch needs improvement:  1
Easy pickings:  0|UI/UX:  0
-+-
Changes (by zsiciarz):

 * stage:  Accepted => Design decision needed


Comment:

 I guess it's a DDN. Generally it is even possible to do something like
 that in SQLite:


 {{{
 Company.objects.update(name=F("name") * F("name"))
 }}}

 And the name field after that is "0". This is rather a wider case, whether
 the ORM should allow silly expressions like that, and not only negation of
 non-booleans. It is completely DBMS-dependent and most "true" DB engines
 will barf immediately.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by zsiciarz):

 Apparently various database engines work differently regarding negation of
 non-boolean values. For example on Postgres each of the following queries
 raises an error:

 {{{
 SELECT NOT 5;
 SELECT NOT "5";
 SELECT NOT "5abc";
 }}}

 while on SQLite it happily returns 0. SQLite casts the expression to a
 NUMERIC value when used in a boolean context, so in each case it is
 "truthy".

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by lrekucki):

 Replying to [comment:8 nate_b]:
 > Regarding "Why ~F() instead of not F()": there are two reasons.  One: To
 match with [https://docs.djangoproject.com/en/dev/topics/db/queries
 /#complex-lookups-with-q-objects Q objects]; Two, and more importantly:
 python itself
 [http://docs.python.org/library/operator.html#operator.__not__ does not
 provide a __not__() method for object instances].  So yes, you are
 absolutely right, but I don't think it can be done.

 Ok, now I feel stupid ;)

 >
 > As to your other points, well taken.  I hope my updated patch is an
 improvement.

 The patch looks good. Now we just need to work out the non-boolean cases
 somehow.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by nate_b):

 Regarding use case for negation on non-boolean values: That's a good
 question; I'm not sure that there is one.  I provided that only as a
 stupid example of lack of composability (at the db level negation on non-
 boolean values can work, but that doesn't mean it is useful).  If you can
 propose a test, I would be glad to include it.  Altogether, though, that
 does raise a good question - so far, all {{{F()}}} objects are generally
 composable across all operations, and I would not want to break that by
 adding another operation.  Not all computations in general are composable,
 though; should {{{F()}}} object composability reflect that?  Now my head
 hurts.

 Regarding "Why ~F() instead of not F()": there are two reasons.  One: To
 match with [https://docs.djangoproject.com/en/dev/topics/db/queries
 /#complex-lookups-with-q-objects Q objects]; Two, and more importantly:
 python itself
 [http://docs.python.org/library/operator.html#operator.__not__ does not
 provide a __not__() method for object instances].  So yes, you are
 absolutely right, but I don't think it can be done.

 As to your other points, well taken.  I hope my updated patch is an
 improvement.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by lrekucki):

 One more thing worth documenting, due to difference in None and NULL
 semantics:

 {{{#!python
 class A(Model):
   f = NullBooleanField()

 for m in A.objects.all():
   m.f = not m.f; m.save()  # all NULL values are now True

 # while this will leave NULL values unchanged as NOT NULL is NULL
 A.objects.update(f=not F('f'))
 }}}

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  1
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by lrekucki):

 * needs_better_patch:  0 => 1
 * needs_tests:  0 => 1
 * needs_docs:  0 => 1


Comment:

 As a new feature this needs documentation. Also:


   * What's the use case for negation on non-boolean values? How does this
 behave on different DBs (the tests should check that).
   * No tests for compounds statements (should they be boolean only?).
   * Why {{{~F()}}} instead of more Pythonic {{{not F()}}}. After all
 {{{~True}}} is {{{-2}}} not {{{False}}}. You could use {{{not}}} for
 boolean expressions and {{{~}}} for bit negation on fields (which might be
 useful too!).

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by niwi):

 Your solution seems to me pretty good. I had not thought of fields that
 are not Boolean. In fact it was an idea that surely someone could have
 implemented better than me. (just this case).

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by nate_b):

 There are several problems with this patch.

 For example, you should be able to negate compound expressions:

 {{{#!python
 >>> Demo.objects.filter(a_field=~(F('a_field')+1))
 Traceback (most recent call last):
   File "", line 1, in 
 TypeError: bad operand type for unary ~: 'ExpressionNode'
 }}}

 Secondly, there is no indication of negation in the string representation,
 like there is for other operators:
 {{{#!python
 >>> print ~F('field')
 (DEFAULT: )
 >>> print F('field') + F('other')
 (+: (DEFAULT: ), (DEFAULT: ))
 }}}

 Thirdly, {{{tree.Node}}} already supplies a {{{negate()}}} function and a
 {{{negated}}} field.  Why not use those instead?

 I'm not entirely pleased with my alternative as it still doesn't make use
 of {{{connector}}}, but it solves these listed problems.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by niwi):

 * needs_tests:  1 => 0


Comment:

 I modified a little test, to cover this modification. Attached the new
 patch.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
-+-
 Reporter:  niwi |Owner:  nobody
 Type:  New feature  |   Status:  new
Component:  Database layer   |  Version:  SVN
  (models, ORM)  |   Resolution:
 Severity:  Normal   | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  1|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by lukeplant):

 * component:  Uncategorized => Database layer (models, ORM)
 * needs_tests:  0 => 1
 * stage:  Unreviewed => Accepted


Comment:

 Seems reasonable, and I can't think of an alternative use of negating an F
 object.

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Re: [Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
---+--
 Reporter:  niwi   |Owner:  nobody
 Type:  New feature|   Status:  new
Component:  Uncategorized  |  Version:  SVN
 Severity:  Normal |   Resolution:
 Keywords: | Triage Stage:  Unreviewed
Has patch:  1  |  Needs documentation:  0
  Needs tests:  0  |  Patch needs improvement:  0
Easy pickings:  0  |UI/UX:  0
---+--
Changes (by niwi):

 * needs_better_patch:   => 0
 * has_patch:  0 => 1
 * needs_tests:   => 0
 * needs_docs:   => 0


Comment:

 I forgot this:

 The result of the sql would look like:

 {{{#!sql
 UPDATE "testmodel" SET "done" = NOT "testmodel"."done";
 }}}

 (tested on sqlite3, postgresql and mysql)

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[Django] #17186: Inverted F expression (negation)

[Django]

#17186: Inverted F expression (negation)
---+
 Reporter:  niwi   |  Owner:  nobody
 Type:  New feature| Status:  new
Component:  Uncategorized  |Version:  SVN
 Severity:  Normal |   Keywords:
 Triage Stage:  Unreviewed |  Has patch:  0
Easy pickings:  0  |  UI/UX:  0
---+
 By default, the expression F django, can not make something like:

 {{{#!sql
 UPDATE "testmodel" SET "done" = NOT "testmodel"."done";
 }}}

 The idea is to make updates or queries like this:

 {{{#!python
 TestModel.objects.update(done=~F('done'))
 }}}

 Taking as a reference, this model:

 {{{#!python
 class TestModel(models.Model):
 done = models.BooleanField(default=False)
 }}}

 Attached is a patch that works for me, but not if it's the best way to do
 it.

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