"""Add a short ``code`` column to ``asset_legal_entities`` so the
public-facing 公司主体信息 admin tab can stamp every entity with
a 1-8-char identifier (e.g. ``CN``, ``US``, ``SG``, ``JP``)
suitable for embedding in machine-readable strings.

**Why now**: the new asset-tag auto-generation rule shipped in this
PR is ``IT-{purchase_entity_code}-{category_code_without_leading_P}-
{YYYYMMDDHHMMSS}`` (example: ``IT-CN-NB-20260508151325``) and
needs a stable short code per legal entity. ``country_or_region``
is already on the table but is descriptive prose ("中国 / China")
not a tag-safe slug; reusing it would couple the tag format to
free-text admin input that's allowed to contain slashes / spaces /
non-ASCII. A dedicated column lets admins choose the tag prefix
deliberately and keeps the tag-format invariants in one place.

**Schema**: nullable so existing rows backfill as NULL — the
intake form's tag generation rejects (with a clear 422) when the
selected purchase_entity has no ``code``, so admins are nudged to
fill it in via the OfficeAddress 公司主体信息 tab on first use.

**Uniqueness**: codes SHOULD be unique per active entity
(otherwise two entities both stamp ``CN`` and the resulting tag
prefix is ambiguous). Enforced by a unique index on
``(code)`` where ``code IS NOT NULL`` — MySQL/TiDB treat NULL
values as distinct in unique indexes, so existing NULL rows
coexist freely until admins fill them in.

Schema-only; no data backfill.
"""
from __future__ import annotations

import sqlalchemy as sa

from alembic import op

revision = "132"
down_revision = "131"
branch_labels = None
depends_on = None


def upgrade() -> None:
    op.add_column(
        "asset_legal_entities",
        sa.Column("code", sa.String(16), nullable=True),
    )
    op.create_index(
        "uq_asset_legal_entities_code",
        "asset_legal_entities",
        ["code"],
        unique=True,
    )


def downgrade() -> None:
    op.drop_index(
        "uq_asset_legal_entities_code",
        table_name="asset_legal_entities",
    )
    op.drop_column("asset_legal_entities", "code")