[Django] #28888: Index added to _meta.indexes with Meta.indexes=[] yields two equal addIndex() operations.

[Django]

#28888: Index added to _meta.indexes with Meta.indexes=[] yields two equal
addIndex() operations.
--------------------------------------------------+------------------------
               Reporter:  Jan Pieter Waagmeester  |          Owner:  nobody
                   Type:  Uncategorized           |         Status:  new
              Component:  Migrations              |        Version:  1.11
               Severity:  Normal                  |       Keywords:
           Triage Stage:  Unreviewed              |      Has patch:  0
    Needs documentation:  0                       |    Needs tests:  0
Patch needs improvement:  0                       |  Easy pickings:  0
                  UI/UX:  0                       |
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 I use a custom field derived from
 `django.contrib.postgres.fields.JSONField`, which adds an `GinIndex` to
 `model._meta.indexes`:
 {{{
 from django.contrib.postgres.fields import JSONField
 from django.contrib.postgres.indexes import GinIndex

 class CustomJSONField(JSONField):
     def contribute_to_class(self, cls, name):
         super(CustomJSONField, self).contribute_to_class(cls, name)

         index = GinIndex(fields=[name])
         index.set_name_with_model(cls)
         cls._meta.indexes.append(index)
 }}}

 When used in a model like this,
 {{{
 class Blog(models.Model):
     title = models.CharField(max_length=100)
     json = CustomJSONField()
 }}}

 Migrations for model and index are created as expected:
 {{{
 ./manage.py --version
 1.11.8
 ./manage.py makemigrations app
 Migrations for 'app':
   app/migrations/0001_initial.py
     - Create model Blog
     - Create index app_blog_json_2cf556_gin on field(s) json of model blog
 }}}

 But when I add an empty list of indexes like this:

 {{{
 class Blog(models.Model):
     title = models.CharField(max_length=100)
     json = CustomJSONField()

     class Meta:
         indexes = []
 }}}
 two indexes are created:

 {{{
 rm -rf app/migrations
 ./manage.py --version
 1.11.8
 ./manage.py makemigrations app
 Migrations for 'app':
   app/migrations/0001_initial.py
     - Create model Blog
     - Create index app_blog_json_2cf556_gin on field(s) json of model blog
     - Create index app_blog_json_2cf556_gin on field(s) json of model blog
 }}}

 Which of course results in `django.db.utils.ProgrammingError: relation
 "app_blog_json_2cf556_gin" already exists`.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/28888>
Django <https://code.djangoproject.com/>
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