"""Add umbral_name to users table.

3-step migration: add nullable → backfill from username → alter to NOT NULL.
Backfill uses username || '_' || id as fallback if uniqueness conflicts arise.

Revision ID: 039
Revises: 038
"""
from alembic import op
import sqlalchemy as sa

revision = "039"
down_revision = "038"
branch_labels = None
depends_on = None


def upgrade() -> None:
    # Step 1: Add nullable column
    op.add_column("users", sa.Column("umbral_name", sa.String(50), nullable=True))

    # Step 2: Backfill from username (handles uniqueness conflicts with fallback)
    op.execute("UPDATE users SET umbral_name = username")
    # Fix any remaining NULLs (shouldn't happen, but defensive)
    op.execute(
        "UPDATE users SET umbral_name = username || '_' || id "
        "WHERE umbral_name IS NULL"
    )

    # Step 3: Alter to NOT NULL and add unique index
    op.alter_column("users", "umbral_name", nullable=False)
    op.create_index("ix_users_umbral_name", "users", ["umbral_name"], unique=True)


def downgrade() -> None:
    op.drop_index("ix_users_umbral_name", table_name="users")
    op.drop_column("users", "umbral_name")