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

This solution is easy to understand and works really well for both upgrade and downgrade. I've written this answer in more detailed manner.

Let's say our enum_type looks like this:

enum_type = ('some_value_1', 'some_value_2')

I want to alter enum_type by adding a new enum, so that it becomes like this:

enum_type = ('some_value_1', 'some_value_2', 'new_value')

This can be done in this way:

from alembic import op


def upgrade():
    op.execute("COMMIT")
    op.execute("ALTER TYPE enum_type ADD VALUE 'new_value'")


def downgrade():
    # Drop 'new_value' from enum_type
    op.execute("ALTER TYPE enum_type RENAME TO enum_type_tmp")

    op.execute("CREATE TYPE enum_type AS ENUM('some_value_1', 'some_value_1')")

    op.execute("DROP TYPE enum_type_tmp")

NOTE: During downgrade, if you're using enum_type in a table then you can modify the downgrade method as mentioned below:

def downgrade():
    # Drop 'new_value' from enum_type
    op.execute("UPDATE table_name"
               " SET column_name_using_enum_type_value = NULL"
               " WHERE column_name_using_enum_type_value = 'new_value'")    

    op.execute("ALTER TYPE enum_type RENAME TO enum_type_tmp")

    op.execute("CREATE TYPE enum_type AS ENUM('some_value_1', 'some_value_1')")

    op.execute("ALTER TABLE table_name"
               " ALTER COLUMN column_name_using_enum_type_value TYPE enum_type"
               " USING column_name_using_enum_type_value::text::enum_type")

    op.execute("DROP TYPE enum_type_tmp")