Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:  fixed
 Severity:  Release blocker  | 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 Loic Bistuer ):

 In [changeset:"91fce675a453764cb233e53c0460826600c828fa"]:
 {{{
 #!CommitTicketReference repository=""
 revision="91fce675a453764cb233e53c0460826600c828fa"
 Use 'update_fields' in RelatedManager.clear() when bulk=False.

 Thanks Simon Charette for the suggestion.

 Refs #21169.
 }}}

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:  fixed
 Severity:  Release blocker  | 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 Anssi Kääriäinen ):

 In [changeset:"01e8ac47b37ccf1b4d9b9710ae2d9b6b486c3c7a"]:
 {{{
 #!CommitTicketReference repository=""
 revision="01e8ac47b37ccf1b4d9b9710ae2d9b6b486c3c7a"
 PEP-8 cleanup

 Refs #21169
 }}}

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:  fixed
 Severity:  Release blocker  | 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 Anssi Kääriäinen ):

 In [changeset:"52015b963d263642957aec880b52ad4063b484cd"]:
 {{{
 #!CommitTicketReference repository=""
 revision="52015b963d263642957aec880b52ad4063b484cd"
 Used simpler queries for m2m clearing when possible.

 Refs #21169
 }}}

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:  fixed
 Severity:  Release blocker  | 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 Anssi Kääriäinen ):

 In [changeset:"f450bc9f44bc1270eae911daa157c155c29d1d9d"]:
 {{{
 #!CommitTicketReference repository=""
 revision="f450bc9f44bc1270eae911daa157c155c29d1d9d"
 Added a bulk option to RelatedManager remove() and clear() methods

 Refs #21169
 }}}

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:  fixed
 Severity:  Release blocker  | 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 Anssi Kääriäinen ):

 * status:  assigned => closed
 * resolution:   => fixed


Comment:

 In [changeset:"17c3997f6828e88e4646071a8187c1318b65597d"]:
 {{{
 #!CommitTicketReference repository=""
 revision="17c3997f6828e88e4646071a8187c1318b65597d"
 Fixed #21169 -- Reworked RelatedManager methods use default filtering

 The `remove()` and `clear()` methods of the related managers created by
 `ForeignKey`, `GenericForeignKey`, and `ManyToManyField` suffered from a
 number of issues. Some operations ran multiple data modifying queries
 without
 wrapping them in a transaction, and some operations didn't respect default
 filtering when it was present (i.e. when the default manager on the
 related
 model implemented a custom `get_queryset()`).

 Fixing the issues introduced some backward incompatible changes:

 - The implementation of `remove()` for `ForeignKey` related managers
 changed
   from a series of `Model.save()` calls to a single `QuerySet.update()`
 call.
   The change means that `pre_save` and `post_save` signals aren't called
 anymore.

 - The `remove()` and `clear()` methods for `GenericForeignKey` related
   managers now perform bulk delete so `Model.delete()` isn't called
 anymore.

 - The `remove()` and `clear()` methods for `ManyToManyField` related
   managers perform nested queries when filtering is involved, which may
   or may not be an issue depending on the database and the data itself.

 Refs. #3871, #21174.

 Thanks Anssi Kääriäinen and Tim Graham for the reviews.
 }}}

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | 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 akaariai):

 The plan is to introduce pre/post update signals, and after that merge the
 changes from this ticket in.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | 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 loic84):

 * needs_docs:  1 => 0


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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  1
Has patch:  1|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by loic84):

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


Comment:

 For the record I'm going to list all the changes and issues addressed by
 this ticket and #21174 since they are related.

 The culprits: the `clear()` and `remove()` methods of the M2M, FK, and GFK
 related managers, so six methods in total.

 Issues:

 - All methods except `FK.clear()` completely bypassed default filtering
 (i.e. custom `Manager.get_queryset()` with filters).
 - All methods except `FK.clear()` caused multiple consecutive db
 operations run without transaction. (see #21174)
 - Inconsistency between the implementation of `FK.remove()` and
 `FK.clear()`, in particular wrt emitted signals.
 - `M2M.clean()` and `M2M.remove()` were prone to race conditions for
 symmetrical relations. (see #comment:8)

 List of changes:

 - All methods now respect default filtering, which means you can't delete
 or change something that is filtered out.

 - All methods now run in a single query, which addresses all transaction
 concerns.

 - `FK.remove()` changed from successive `save()` calls, to a single
 `update()`.
 Gotcha: `pre_save` and `post_save` signals aren't fired anymore.

 - `M2M.remove()` and `M2M.clear()` rely on subqueries when filtering is
 involved.

 - Symmetrical `M2M.remove()` and `M2M.clear()` now run in a single query.

 - `GFK.remove()` and `GFK.clear()` now perform bulk delete.
 Gotcha: `Model.delete()` is not called anymore.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by akaariai):

 I did some work to make sure queries used by m2m clear() and remove() are
 as simple as possible. Now the clearing queries do not use subqueries when
 the manager doesn't have filters. The subqueries in these cases didn't
 actually filter anything out. In addition, when subqueries are used, the
 subquery has one less join than before. End result is that the queries are
 similar to current master when the base manager doesn't have filters, and
 a bit more efficient that current HEAD of pull 1685 when there are
 filters. The tradeoff is even more complexity in m2m manager
 implementation.

 To me it seems important that as simple as possible queries are used. This
 ensures as little DB resources are used as possible. In addition I believe
 old versions of MySQL will simply panic when they see a query with both a
 subquery and joins...

 Patch at
 
https://github.com/akaariai/django/commit/9dd75213050e6f85a0b93ddd59243f8d3809b06c
 - this doesn't have any tests, and regressions in query efficiency seem
 likely. I wonder if CaptureQueriesContext could be used to inspect the
 used queries.

 BTW I spotted one possible data-corruption issue in current clear() code
 for symmetric m2m. Currently the clear is implemented as two separate
 delete queries which delete all items from the relation, first in one
 direction, then in another. But, if a concurrent add() commits in between
 then the added relation is deleted in one direction, but not in anohter.
 The sequence is like this for transactions T1 and T2:
 {{{
 T1: add relation for instances m1 <-> m2. Two rows are added so that the
 relation is symmetrical (rows m1:m2 and m2:m1).
 T2: m1.clear() starts. First, delete all rows where m1:*. m1:m2 isn't
 deleted as that isn't yet visible.
 T1: commits, m1:m2 and m2:m1 become visible to T2.
 T2: Second part of clear is executed, that is *:m1 is deleted. Now m2:m1
 is visible and it is deleted.
 T2: commits. m1:m2 relation exists, but m2:m1 doesn't. The symmetry of
 m1<->m2 is broken.
 }}}
 Luckily this seems rare in practice. pull 1685 does the delete in one go,
 so this issue is fixed in PR1685. This seems to be impossible to test in a
 way that the regression test actually picks up regressions. So, no tests
 needed for this case.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by akaariai):

 Just a note to those who try to fix release blockers for 1.6: the release
 blocker is for master, not 1.6.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-

Comment (by loic84):

 I've reworked M2M `RelatedManager.clear()` to account for the default
 filtering through a subquery.

 https://github.com/django/django/pull/1685

 As previously noted, GFK and FK `RelatedManager.clear()` weren't affected
 by the issue.

 Also as discussed on IRC, `remove()` seems less critical since you specify
 the objects manually, so we might get away with a warning in the docs.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  loic84
 Type:  Bug  |   Status:  assigned
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by loic84):

 * owner:  nobody => loic84
 * status:  new => assigned


Comment:

 The problem is not limited to the `__call__` syntax, the same would happen
 to default managers.

 I'm working on a fix.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:  Accepted
 Keywords:   |  Needs documentation:  0
Has patch:  0|  Patch needs improvement:  0
  Needs tests:  0|UI/UX:  0
Easy pickings:  0|
-+-
Changes (by akaariai):

 * stage:  Unreviewed => Accepted


Comment:

 Hmmh, lets just go for "forbid operations" approach. These methods can be
 added back later on if we can fix them in clean way.

 Actually, lets go for "no related manager methods" approach. You can also
 do __set__ which could clear existing instances outside the manager's
 viewable set, and .add() could add something that isn't viewable by the
 manager.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:
 Keywords:   |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by sebastian):

 Unfortunately, I am not entirely sure how to proceed here. When I wrote my
 patch proposal for #3871, I remember it wasn't possible to make `remove()`
 and many-to-many's `clear()` work as expected without any additional
 overhead (necessary to make sure so that we do not remove more objects
 from the relation as intended).

 That is the reason why I decided to entirely forbid `remove()` and many-
 to-many's `clear()` when their outcome would be unexpected. Is this a
 reasonable approach here, as well? Ideally, providing these methods would
 be great but are they even possible without issuing another query (i.e.,
 to make sure that the object removed is actually part of the collection in
 case of `remove()`)?

 Due to the way it is implemented, foreign-key's `clear()` already works as
 expected. So the problem lies only in foreign-key's and many-to-many's
 `remove()` and many-to-many's `clear()`.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:
 Keywords:   |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-

Comment (by akaariai):

 I think I am going to revert #3871, we can then improve the patch and
 recommit. Of course, if we can solve this in agreed upon way really soon
 there is no need to revert + repatch.

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Re: [Django] #21169: Deletion in custom reverse managers

[Django]

#21169: Deletion in custom reverse managers
-+-
 Reporter:  sebastian|Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  master
  (models, ORM)  |   Resolution:
 Severity:  Release blocker  | Triage Stage:
 Keywords:   |  Unreviewed
Has patch:  0|  Needs documentation:  0
  Needs tests:  0|  Patch needs improvement:  0
Easy pickings:  0|UI/UX:  0
-+-
Changes (by sebastian):

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


Comment:

 In my patch proposal patch `r17204-custom-reverse-managers.diff` in #3871,
 I considered the problem and then took the easy way out by simply
 disallowing the `remove()` and `clear()` methods on custom reverse
 managers. Obviously, this solution is far from perfect.

-- 
Ticket URL: 
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