id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
25678	migrate scalar to array field in PostrgreSQL fails	Mark Mikofski	nobody	"Django-1.8.5
Python 2.7.10
PostgreSQL-9.4

Problem: Scalar type might not be cast to array of same type explicitly with `USING` SQL command?

Solution: Use psql to migrate fields manually
{{{
dev=# ALTER TABLE my_app_mymodel ALTER COLUMN ""my_field"" TYPE double precision [] USING array[""my_field""]::double precision[];
ALTER TABLE
}}}


== Details ==
given an initial model:
{{{
class MyModel(models.Model):
    my_field = models.FloatField()
}}}

change this to `ArrayField`
{{{
class MyModel(models.Model):
    my_field = ArrayField(
        base_field=models.FloatField(),
        default=list
    )
}}}

then make migrations and migrate
{{{
$ ./manage.py makemigrations
$ ./manage.py migrate
}}}

returns the following traceback:
{{{
Operations to perform:
  Synchronize unmigrated apps: staticfiles, messages
  Apply all migrations: admin, contenttypes, my_app, auth, sessions
Synchronizing apps without migrations:
  Creating tables...
    Running deferred SQL...
  Installing custom SQL...
Running migrations:
  Rendering model states... DONE
  Applying my_app.0XYZ_auto_YYYYMMDD_hhmm...Traceback (most recent call last):
  File ""./manage.py"", line 10, in <module>
    execute_from_command_line(sys.argv)
  File ""C:\Python27\lib\site-packages\django\core\management\__init__.py"", line 351, in execute_from_command_line
    utility.execute()
  File ""C:\Python27\lib\site-packages\django\core\management\__init__.py"", line 343, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File ""C:\Python27\lib\site-packages\django\core\management\base.py"", line 394, in run_from_argv
    self.execute(*args, **cmd_options)
  File ""C:\Python27\lib\site-packages\django\core\management\base.py"", line 445, in execute
    output = self.handle(*args, **options)
  File ""C:\Python27\lib\site-packages\django\core\management\commands\migrate.py"", line 222, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File ""C:\Python27\lib\site-packages\django\db\migrations\executor.py"", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File ""C:\Python27\lib\site-packages\django\db\migrations\executor.py"", line 148, in apply_migration
    state = migration.apply(state, schema_editor)
  File ""C:\Python27\lib\site-packages\django\db\migrations\migration.py"", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File ""C:\Python27\lib\site-packages\django\db\migrations\operations\fields.py"", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File ""C:\Python27\lib\site-packages\django\db\backends\base\schema.py"", line 484, in alter_field
    old_db_params, new_db_params, strict)
  File ""C:\Python27\lib\site-packages\django\db\backends\base\schema.py"", line 636, in _alter_field
    params,
  File ""C:\Python27\lib\site-packages\django\db\backends\base\schema.py"", line 111, in execute
    cursor.execute(sql, params)
  File ""C:\Python27\lib\site-packages\django\db\backends\utils.py"", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File ""C:\Python27\lib\site-packages\django\db\backends\utils.py"", line 64, in execute
    return self.cursor.execute(sql, params)
  File ""C:\Python27\lib\site-packages\django\db\utils.py"", line 97, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File ""C:\Python27\lib\site-packages\django\db\backends\utils.py"", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: column ""my_field "" cannot be cast automatically to type double precision[]
HINT:  Specify a USING expression to perform the conversion.
}}}
"	Bug	closed	Migrations	dev	Normal	fixed	PostgreSQL, ArrayField		Accepted	0	0	0	0	0	0