Altering an Enum field using Alembic

Question

How can I add an element to an Enum field in an alembic migration when using a version of PostgreSQL older than 9.1 (which adds the ALTER TYPE for enums)? This SO question e

Answer 1

I used a bit simpler approach with less steps than the accepted answer, which I based this on. In this example I will pretend the enum in question is called 'status_enum', because in the accepted answer the use of 'status' for both the column and enum confused me.

from alembic import op 
import sqlalchemy as sa

name = 'status_enum'
tmp_name = 'tmp_' + name

old_options = ('nonexistent_executable', 'signal', 'success', 'timed_out')
new_options = sorted(old_options + ('output_limit_exceeded',))

new_type = sa.Enum(*new_options, name=name)
old_type = sa.Enum(*old_options, name=name)

tcr = sa.sql.table('testcaseresult',
                   sa.Column('status', new_type, nullable=False))

def upgrade():
    op.execute('ALTER TYPE ' + name + ' RENAME TO ' + tmp_name)

    new_type.create(op.get_bind())
    op.execute('ALTER TABLE testcaseresult ALTER COLUMN status ' +
               'TYPE ' + name + ' USING status::text::' + name)
    op.execute('DROP TYPE ' + tmp_name)


def downgrade():
    # Convert 'output_limit_exceeded' status into 'timed_out'                                                                                                                      
    op.execute(tcr.update().where(tcr.c.status=='output_limit_exceeded')
               .values(status='timed_out'))

    op.execute('ALTER TYPE ' + name + ' RENAME TO ' + tmp_name)

    old_type.create(op.get_bind())
    op.execute('ALTER TABLE testcaseresult ALTER COLUMN status ' +
               'TYPE ' + name + ' USING status::text::' + name)
    op.execute('DROP TYPE ' + tmp_name)