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

[Jan Pieter Waagmeester]

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 to class Meta 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.

[Jan Pieter Waagmeester]

Possibly unrelated, but:

When an initial migration is added without the index added to cls._meta.indexes, resulting in the output

./manage.py makemigrations app
Migrations for 'app':
  app/migrations/0001_initial.py
    - Create model Blog

No index is added, as expected.

However, adding the index to cls._meta.indexes has no effect: No changes detected in app 'app'.

[Tim Graham]

As there's no documented support for adding indexes in Field.contribute_to_class(), can you explain why Django is at fault and/or propose a patch?

[Jan Pieter Waagmeester]

I understand that "there's no documented support for adding indexes in Field.contribute_to_class()" and I'm open to suggestions to other solutions to add indexes with a field.

But this seems the cleanest way to do it, and support seems to be almost there/halfway there. I'm willing to invest time to create a patch to fix this, but I'd like to have some degree of confidence that such a solution would be desirable functionality.

[Tim Graham]

Usually it's more effective to use the DevelopersMailingList to get feedback and consensus on design decisions as it reaches a wider audience than the ticket tracker.