Django and postgres migration errors - django

Question

I have created a new postgressql db in my django app within Heroku. Every time I run a migration, however, I get an error that I don't understand.
Error:
Applying argent.0043_auto_20170322_1629...Traceback (most recent call last):
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
psycopg2.ProgrammingError: cannot cast type date to integer
LINE 1: ...LTER COLUMN "date_id" TYPE integer USING "date_id"::integer,...
I have removed and added the only date field I have, to no avail. I still get the same error whether the date field in my models is there or not. Any insight would be much appreciated.
models.py
class Entry(models.Model):
date = models.DateField(blank=True, null=True,)
euros = models.CharField(max_length=500, blank=True, null=True)
comments = models.CharField(max_length=900, blank=True, null=True)
euros_sum = models.DecimalField(max_digits=6, decimal_places=2, blank=True, null=True)
xrate = models.DecimalField(max_digits=6, decimal_places=2, blank=True, null=True)
dollars_sum = models.DecimalField(max_digits=6, decimal_places=2, blank=True, null=True)
daily_savings_dollars = models.DecimalField(max_digits=6, decimal_places=2, blank=True, null=True)
daily_savings_display = models.DecimalField(max_digits=6, decimal_places=2, blank=True, null=True)
def get_absolute_url(self):
return reverse('argent:detail', kwargs={'pk': self.pk})
def item_date(self):
row_title = self.date
return row_title
class Meta:
ordering = ['date']
traceback:
Traceback (most recent call last):
File "manage.py", line 22, in <module>
execute_from_command_line(sys.argv)
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
utility.execute()
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/__init__.py", line 355, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/base.py", line 283, in run_from_argv
self.execute(*args, **cmd_options)
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/base.py", line 330, in execute
output = self.handle(*args, **options)
File "/app/.heroku/python/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 204, in handle
fake_initial=fake_initial,
File "/app/.heroku/python/lib/python3.6/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 "/app/.heroku/python/lib/python3.6/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 "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
state = migration.apply(state, schema_editor)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/migration.py", line 129, in apply
operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/migrations/operations/fields.py", line 215, in database_forwards
schema_editor.alter_field(from_model, from_field, to_field)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 513, in alter_field
old_db_params, new_db_params, strict)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/postgresql/schema.py", line 112, in _alter_field
new_db_params, strict,
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 674, in _alter_field
params,
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 119, in execute
cursor.execute(sql, params)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/utils.py", line 94, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/app/.heroku/python/lib/python3.6/site-packages/django/utils/six.py", line 685, in reraise
raise value.with_traceback(tb)
File "/app/.heroku/python/lib/python3.6/site-packages/django/db/backends/utils.py", line 65, in execute
return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: cannot cast type date to integer
LINE 1: ...LTER COLUMN "date_id" TYPE integer USING "date_id"::integer,...

Answer 1

If want to migrate the date column from a integer type to to a datetime one, you need to handle any existing entries you have in the table before you can run the migration.
It's likely that you already have some integers in the date column when you try to change the column type, so the conversion fails as postgres doesn't know how to convert an integer to a datetime.
You can either delete all your entries if you don't care about them, or otherwise you need to break you migration into separate steps:
Create the new datetime column with a temporary name
Run a data migration to convert your integers into datetimes and populate the new column
Drop the old integer date column, rename the new one with the final name.