#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 =>


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

 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(
     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
 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.

Ticket URL: <https://code.djangoproject.com/ticket/26352#comment:6>
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