Django Makemigrations don't check if field name from indexes exists on model before creating a migration.

[Gabriel]

When using makemigration with indexes on class Meta in the Model Django don't check whether the field name in fields parameter exists on Model.

Because of this both the migration files (in migrations/*.py) and the model class on model.py will have a index that referrer to a non existing field, so when manage.py migrate is run, the expection FieldDoesNotExist will be raised, but in this moment the programmer will have to edit both the migration file and model class to fix the error, especially if is not possible to remove the migrations file or is more convenient to edit the migration in projects with many Apps and many dependency between those Apps.

Would be more convenient if makemigration before generate the new migration files did this verification.

Example of model's code with wrong field name ("names") on indexes:

class ItemType(models.Model):
	name = models.CharField(_('Name'), unique=True, max_length=255)

	class Meta:
		managed = True
		db_table = 'item_type'
		ordering = ['name']

		indexes = [
			models.Index(fields=['id'], name='item_type_index_1'),
			models.Index(fields=['-id'], name='item_type_index_2'),
			models.Index(fields=['names'], name='item_type_index_3'),
			models.Index(fields=['-names'], name='item_type_index_4')
		]

This code will not raise any exception on makemigrations, only the generated code on migration file will when migrate is used:

       migrations.AddIndex(
            model_name='itemtype',
            index=models.Index(fields=['names'], name='item_type_index_3'),
        ),
        migrations.AddIndex(
            model_name='itemtype',
            index=models.Index(fields=['-names'], name='item_type_index_4'),
        ),

Traceback (manage.py migrate):

Traceback (most recent call last):
  File "D:\Project-2015\cultural\manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "C:\Python36\lib\site-packages\django\core\management\__init__.py", line 364, in execute_from_command_line
    utility.execute()
  File "C:\Python36\lib\site-packages\django\core\management\__init__.py", line 356, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Python36\lib\site-packages\django\core\management\base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\Python36\lib\site-packages\django\core\management\base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "C:\Python36\lib\site-packages\django\core\management\commands\migrate.py", line 204, in handle
    fake_initial=fake_initial,
  File "C:\Python36\lib\site-packages\django\db\migrations\executor.py", line 115, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "C:\Python36\lib\site-packages\django\db\migrations\executor.py", line 145, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "C:\Python36\lib\site-packages\django\db\migrations\executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\Python36\lib\site-packages\django\db\migrations\migration.py", line 129, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "C:\Python36\lib\site-packages\django\db\migrations\operations\models.py", line 788, in database_forwards
    schema_editor.add_index(model, self.index)
  File "C:\Python36\lib\site-packages\django\db\backends\base\schema.py", line 331, in add_index
    self.execute(index.create_sql(model, self))
  File "C:\Python36\lib\site-packages\django\db\models\indexes.py", line 65, in create_sql
    sql_parameters = self.get_sql_create_template_values(model, schema_editor, using)
  File "C:\Python36\lib\site-packages\django\db\models\indexes.py", line 48, in get_sql_create_template_values
    fields = [model._meta.get_field(field_name) for field_name, order in self.fields_orders]
  File "C:\Python36\lib\site-packages\django\db\models\indexes.py", line 48, in <listcomp>
    fields = [model._meta.get_field(field_name) for field_name, order in self.fields_orders]
  File "C:\Python36\lib\site-packages\django\db\models\options.py", line 619, in get_field
    raise FieldDoesNotExist("%s has no field named '%s'" % (self.object_name, field_name))
django.core.exceptions.FieldDoesNotExist: ItemType has no field named 'names'

Attached is this model example with a incorrect index field name that will not raise exception on makemigration only on migrate.

This was checked on Django 1.11.5 with Python 3.6.2 on Windows 10 64bits.

To reproduce this bug:

1. Add the Item App to a existing Django project
2. Run manage.py makemigrations item
3. Run manage.py migrate

[Gabriel]

Folder with App named item.