django changing a date field to integer field can't migrate

Question

I recently changed a date field to an integer field (the data was specified in number of months remaining rather than a date).

However all though the make migrations command works fine when I attempt to migrate this fails with a typecast error (note using postgres 9.5). Note there was only 1 instance/entry of the model, which I have also now deleted and the migrate still fails for some reason?

File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: cannot cast type date to integer
LINE 1: ...end_date_fixed" TYPE integer USING "end_date_fixed"::integer
                                                              ^


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/core/management/__init__.py", line 353, in execute_from_command_line
    utility.execute()
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/core/management/__init__.py", line 345, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/core/management/commands/migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/migrations/executor.py", line 92, in migrate
    self._migrate_all_forwards(plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/migrations/executor.py", line 121, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/migrations/executor.py", line 198, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/migrations/migration.py", line 123, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 482, in alter_field
    old_db_params, new_db_params, strict)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/postgresql/schema.py", line 110, in _alter_field
    new_db_params, strict,
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 634, in _alter_field
    params,
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 110, in execute
    cursor.execute(sql, params)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/utils.py", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/Users/Yunti/.virtualenvs/switcher5/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: cannot cast type date to integer
LINE 1: ...end_date_fixed" TYPE integer USING "end_date_fixed"::integer

Answer 1

If the table is empty.

Do the migration in two simple steps. Step 1, drop the existing date column from the model. Step 2 add a new INTEGER column with the same column name.

If the table is not empty.

Edit your model to change the type of end_date_fixed to integer as you have done. Do the make migrations, the open the migrations and delete everything inside migrations[] then replacce it with

 migrations.RunSQL("ALTER TABLE mytable ALTER end_date_fixed TYPE INTEGER USING EXTRACT(epoch FROM end_date_fixed)

Make sure that you have not made any other changes to the model when you try this method.