"""referrals: user.referral_code + user.referred_by_user_id + referrals table

Phase D.1 of the multi-user billing work. Adds:

- `users.referral_code` — unique 8-char URL-safe code per user, generated
  lazily on first visit to /settings (or signup).
- `users.referred_by_user_id` — FK to the user who referred this account,
  set at signup if `?ref=<code>` was supplied. Null otherwise.
- `referrals` — audit trail. One row per (referrer, referred) pair when the
  link is captured. `converted_at` / `credited_at` filled in D.3 by the
  Paddle webhook when the referred user makes their first paid subscription.

The Credit table that holds actual discount records is deferred to D.3 —
no point creating it until Paddle is wired and we know what to write.

Revision ID: 0013
Revises: 0012
Create Date: 2026-05-18
"""
from typing import Sequence, Union

import sqlalchemy as sa
from alembic import op


revision: str = "0013"
down_revision: Union[str, None] = "0012"
branch_labels: Union[str, Sequence[str], None] = None
depends_on: Union[str, Sequence[str], None] = None


def upgrade() -> None:
    # batch_alter_table wraps ADD CONSTRAINT in a copy-and-rename for
    # SQLite (no native ALTER constraints support); on MariaDB/Postgres
    # it falls through to plain ALTER statements.
    with op.batch_alter_table("users") as bop:
        bop.add_column(sa.Column("referral_code", sa.String(16), nullable=True))
        bop.create_unique_constraint(
            "uq_users_referral_code", ["referral_code"],
        )
        bop.add_column(sa.Column("referred_by_user_id", sa.Integer, nullable=True))
        bop.create_foreign_key(
            "fk_users_referred_by",
            "users",
            ["referred_by_user_id"], ["id"],
            ondelete="SET NULL",
        )

    op.create_table(
        "referrals",
        sa.Column("id", sa.BigInteger, primary_key=True, autoincrement=True),
        sa.Column("referrer_user_id", sa.Integer,
                  sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
        # UNIQUE — a single user can only be referred once, ever.
        sa.Column("referred_user_id", sa.Integer,
                  sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
        # converted_at = referred user made their first paid sub. credited_at =
        # we successfully applied the discount via Paddle. Both filled in D.3.
        sa.Column("converted_at", sa.DateTime(timezone=True)),
        sa.Column("credited_at", sa.DateTime(timezone=True)),
        sa.UniqueConstraint("referred_user_id", name="uq_referrals_referred"),
    )
    op.create_index(
        "ix_referrals_referrer", "referrals", ["referrer_user_id"],
    )


def downgrade() -> None:
    op.drop_index("ix_referrals_referrer", table_name="referrals")
    op.drop_table("referrals")
    with op.batch_alter_table("users") as bop:
        bop.drop_constraint("fk_users_referred_by", type_="foreignkey")
        bop.drop_column("referred_by_user_id")
        bop.drop_constraint("uq_users_referral_code", type_="unique")
        bop.drop_column("referral_code")