Title: Sqlalchemy enum migration update fails saying does not exist
Tags: python;postgresql;enums;sqlalchemy;flask-sqlalchemy
Question: I have following sqlalchemy model:

```class Cart(db.Model):
    __tablename__ = 'carts'
    #...
    cart_status = db.Column(db.Enum('confirmed', 'canceled', name='cart_statuses'))
```

Which generates following migration script:

```"""empty message

Revision ID: c7cbe7d1d686
Revises: 56e9612a77ee
Create Date: 2017-06-21 08:52:00.987769

"""
from alembic import op
import sqlalchemy as sa


# revision identifiers, used by Alembic.
revision = 'c7cbe7d1d686'
down_revision = '56e9612a77ee'
branch_labels = None
depends_on = None


def upgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.add_column('carts', sa.Column('cart_status', sa.Enum('confirmed', 'canceled', name='cart_statuses'), nullable=True))
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('carts', 'cart_status')
    # ### end Alembic commands ###
```

When I try to upgrade, I get following error:

```(ecom_bot) root@logicandthoughts:~/ecom/ecombot# python manage.py db upgrade
INFO  [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO  [alembic.runtime.migration] Will assume transactional DDL.
INFO  [alembic.runtime.migration] Running upgrade 56e9612a77ee -&gt; c7cbe7d1d686, empty message
Traceback (most recent call last):
  File "manage.py", line 37, in <module&gt;
    manager.run()
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/flask_script/__init__.py", line 412, in run
    result = self.handle(sys.argv[0], sys.argv[1:])
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/flask_script/__init__.py", line 383, in handle
    res = handle(*args, **config)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/flask_script/commands.py", line 216, in __call__
    return self.run(*args, **kwargs)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/flask_migrate/__init__.py", line 247, in upgrade
    command.upgrade(config, revision, sql=sql, tag=tag)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/command.py", line 254, in upgrade
    script.run_env()
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/script/base.py", line 421, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/util/pyfiles.py", line 93, in load_python_file
    module = load_module_py(module_id, path)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/util/compat.py", line 75, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "migrations/env.py", line 87, in <module&gt;
    run_migrations_online()
  File "migrations/env.py", line 80, in run_migrations_online
    context.run_migrations()
  File "<string&gt;", line 8, in run_migrations
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/runtime/environment.py", line 817, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/runtime/migration.py", line 329, in run_migrations
    step.migration_fn(**kw)
  File "/root/ecom/ecombot/migrations/versions/c7cbe7d1d686_.py", line 21, in upgrade
    op.add_column('carts', sa.Column('cart_status', sa.Enum('confirmed', 'canceled', name='cart_statuses'), nullable=True))
  File "<string&gt;", line 8, in add_column
  File "<string&gt;", line 3, in add_column
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/operations/ops.py", line 1551, in add_column
    return operations.invoke(op)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/operations/base.py", line 318, in invoke
    return fn(self, operation)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/operations/toimpl.py", line 123, in add_column
    schema=schema
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 172, in add_column
    self._exec(base.AddColumn(table_name, column, schema=schema))
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 118, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 945, in execute
    return meth(self, multiparams, params)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1002, in _execute_ddl
    compiled
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1189, in _execute_context
    context)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1402, in _handle_dbapi_exception
    exc_info
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 203, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1182, in _execute_context
    context)
  File "/root/ecom/ecom_bot/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 470, in do_execute
    cursor.execute(statement, parameters)
sqlalchemy.exc.ProgrammingError: (psycopg2.ProgrammingError) type "cart_statuses" does not exist
LINE 1: ALTER TABLE carts ADD COLUMN cart_status cart_statuses
                                                 ^
 [SQL: 'ALTER TABLE carts ADD COLUMN cart_status cart_statuses']
```


Here is the accepted answer: I had to update to following:

```"""empty message

Revision ID: 51aa3bff68d6
Revises: c7cbe7d1d686
Create Date: 2017-06-21 09:02:55.252361

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.dialects import postgresql

# revision identifiers, used by Alembic.
revision = '51aa3bff68d6'
down_revision = 'c7cbe7d1d686'
branch_labels = None
depends_on = None


def upgrade():
    cart_status = postgresql.ENUM('user_unconfirmed', 'user_confirmed', 'client_unconfirmed', 'client_confirmed', name='cart_status')
    cart_status.create(op.get_bind())


    # ### commands auto generated by Alembic - please adjust! ###
    op.add_column('carts', sa.Column('cart_status', sa.Enum('user_unconfirmed', 'user_confirmed', 'client_unconfirmed', 'client_confirmed', name='cart_status'), nullable=True))
    # ### end Alembic commands ###


def downgrade():
    # ### commands auto generated by Alembic - please adjust! ###
    op.drop_column('carts', 'cart_status')
    # ### end Alembic commands ###
    cart_status = postgresql.ENUM('user_unconfirmed', 'user_confirmed', 'client_unconfirmed', 'client_confirmed', name='cart_status')
    cart_status.drop(op.get_bind())
```

Comment for this answer: Any idea why this error would appear suddenly? I ran one migration with an enum field and it worked just fine. When I went to run another migration that simply renamed the enum and the column name, it gave me this error. 

However, following the instructions in this answer did fix the migration. So yay for good answers!


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