Squashing migrations in which unique=True is added and then removed for Charfield cannot migrate backwards on Postgres

[Linus Schoemaker]

This bug can be reproduced as follows:

• Create a model with a CharField with unique=True
• Create a migration
• Alter the model so that CharField is no longer unique
• Create a new migration
• Squash the two migrations
• Migrate forwards, and then backwards

Error output:

psycopg2.ProgrammingError: relation "survey_respondent_email_b0f1dc87_like" already exists


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 "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/core/management/__init__.py", line 353, in execute_from_command_line
    utility.execute()
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/core/management/__init__.py", line 345, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/core/management/base.py", line 348, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/core/management/base.py", line 399, in execute
    output = self.handle(*args, **options)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/core/management/commands/migrate.py", line 200, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/executor.py", line 96, in migrate
    self._migrate_all_backwards(plan, full_plan, fake=fake)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/executor.py", line 163, in _migrate_all_backwards
    self.unapply_migration(states[migration], migration, fake=fake)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/executor.py", line 218, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/migration.py", line 172, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/operations/fields.py", line 206, in database_backwards
    self.database_forwards(app_label, schema_editor, from_state, to_state)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 482, in alter_field
    old_db_params, new_db_params, strict)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/postgresql/schema.py", line 116, in _alter_field
    self.execute(like_index_statement)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/base/schema.py", line 110, in execute
    cursor.execute(sql, params)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/utils.py", line 79, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/utils.py", line 95, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/var/lib/jenkins/shiningpanda/jobs/914dc536/virtualenvs/d41d8cd9/lib/python3.4/site-packages/django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "survey_respondent_email_b0f1dc87_like" already exists

This issue was not present in version 1.9.0, but appeared in version 1.9.2 (or 1.9.1). Probably related to #26034

[Tim Graham]

I followed those steps but couldn't reproduce. Could list the exact commands for each step to ensure I didn't misinterpret?