Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
--+
 Reporter:  Simon Willison|Owner:  nobody
 Type:  Bug   |   Status:  closed
Component:  Core (System checks)  |  Version:  1.9
 Severity:  Normal|   Resolution:  fixed
 Keywords:| Triage Stage:  Accepted
Has patch:  1 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  1
Easy pickings:  0 |UI/UX:  0
--+
Changes (by Tim Graham ):

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


Comment:

 In [changeset:"586a9dc4295357de1f5ad0590ad34bf2bc008f79" 586a9dc4]:
 {{{
 #!CommitTicketReference repository=""
 revision="586a9dc4295357de1f5ad0590ad34bf2bc008f79"
 Fixed #26352 -- Made system check allow ManyToManyField to target the same
 model if through_fields differs.
 }}}

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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
--+
 Reporter:  Simon Willison|Owner:  nobody
 Type:  Bug   |   Status:  new
Component:  Core (System checks)  |  Version:  1.9
 Severity:  Normal|   Resolution:
 Keywords:| Triage Stage:  Accepted
Has patch:  1 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  1
Easy pickings:  0 |UI/UX:  0
--+

Comment (by Simon Willison):

 I've opened a new pull request that applies this change against the latest
 master of Django 2.x

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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
--+
 Reporter:  Simon Willison|Owner:  nobody
 Type:  Bug   |   Status:  new
Component:  Core (System checks)  |  Version:  1.9
 Severity:  Normal|   Resolution:
 Keywords:| Triage Stage:  Accepted
Has patch:  1 |  Needs documentation:  0
  Needs tests:  0 |  Patch needs improvement:  1
Easy pickings:  0 |UI/UX:  0
--+
Changes (by Tim Graham):

 * needs_better_patch:  0 => 1
 * component:  Database layer (models, ORM) => Core (System checks)
 * stage:  Unreviewed => Accepted


Comment:

 Does the original pull request solve it? If so, feel free to update and
 resend it.

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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  Simon Willison   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 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 Raymond Penners):

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


Comment:

 This ticket got closed because of a missing use case. For what it is
 worth, our "real world use case" is the following:

 {{{#!python
 class ShippingMethod(models.Model):
 
 to_countries = models.ManyToManyField(
 Country, through='carriers.ShippingMethodPrice',
 through_fields=('method', 'to_country'))

 # This is the 2nd ManyToManyField causing E003:
 #
 # from_countries = models.ManyToManyField(
 # Country, through='carriers.ShippingMethodPrice',
 # through_fields=('method', 'from_country'), related_name='+')


 class ShippingMethodPrice(models.Model):
 ...
 method = models.ForeignKey(
 ShippingMethod,
 related_name='prices'
 )
 to_country = models.ForeignKey(Country)
 from_country = models.ForeignKey(Country, related_name='+')
 price = models.DecimalField()

 }}}

 So, basically, we have list of shipping methods, and each shipping method
 has a price list, containing a specific price per from/to-country
 combination.
 Given a method, `method.from_countries` should represent the set of all
 countries that are occur in the price list as a from-country.

 I do not think the workaround suggested in
 https://code.djangoproject.com/ticket/26352#comment:2 works for us, as our
 case involves 3 models.

 The hack to disable E003 seems to work, though a real fix (or suggestions
 how to receive the same result otherwise) would be appreciated.

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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  simonw   |Owner:  nobody
 Type:  Bug  |   Status:  closed
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 Severity:  Normal   |   Resolution:  wontfix
 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 timgraham):

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


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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  simonw   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 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
-+-

Comment (by timgraham):

 In absence of a reported use case, I don't mind closing the issue for now.

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Ticket URL: 
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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  simonw   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 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
-+-

Comment (by charettes):

 The only use-case I can think of right now is working around limitations
 of reverse relationships (#897 seems to mention this exact workaround).

 If we ever unify the reverse relationships API by exposing them as normal
 fields I think an explicit way of defining them would be necessary (to
 define options such as `verbose_name`).

 In the meantime I'm not sure about what should be done here as I vaguely
 remember relying on a similar field definition in the past and it
 definitely worked in Django 1.2. On the other hand I wouldn't be surprised
 if it breaks Django in subtle ways as it seems untested (migrations come
 to mind here).

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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  simonw   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 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
-+-

Comment (by simonw):

 As Simon Charette pointed out in a comment on the pull request, this may
 not be necessary. In the above example the same effect can be achieved
 using the related_name property, like this:

 {{{#!python
 class User(models.Model):
 friends = models.ManyToManyField(
 'self', through='Followship', symmetrical=False,
 through_fields=('user', 'target'),
 related_name='followers'
 )
 }}}

 In which case, is there a real world use-case in which it's useful to be
 able to specify multiple ManyToManyFields in the way proposed by this
 ticket?

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Ticket URL: 
Django 
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Re: [Django] #26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self that differ by through_fields

[Django]

#26352: models.E003 check incorrectly prevents duplicate ManyToMany through-self
that differ by through_fields
-+-
 Reporter:  simonw   |Owner:  nobody
 Type:  Bug  |   Status:  new
Component:  Database layer   |  Version:  1.9
  (models, ORM)  |
 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 simonw):

 * has_patch:  0 => 1


Comment:

 I've posted a pull request with a potential fix to Github:
 https://github.com/django/django/pull/6295

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