"""
Lightweight, idempotent schema top-ups.

`db.create_all()` creates missing tables but never alters existing ones, so a
new column on an existing table needs help. This adds only what's missing and
is safe to run on every boot, on both SQLite and Postgres.

It is not a replacement for real migrations — it handles additive column
changes, which is all LIM has needed so far.

Created by: Jess Rogerson (yelling commands at Claude.AI)
"""

from sqlalchemy import inspect, text

from app import db

# table -> column -> DDL fragment used in ALTER TABLE ... ADD COLUMN
ADDITIVE_COLUMNS = {
    'sets': {
        # Existing rows keep the behaviour they had before visibility existed:
        # everyone could see every set, so they become 'public' rather than
        # silently vanishing from other users' libraries. New sets use the
        # site default instead.
        'visibility': "VARCHAR(10) NOT NULL DEFAULT 'public'",
    },
}


def ensure_schema(app):
    """Add any missing columns. Returns a list of changes made."""
    applied = []
    with app.app_context():
        try:
            inspector = inspect(db.engine)
            existing_tables = set(inspector.get_table_names())
        except Exception as exc:  # database not reachable yet
            app.logger.warning('Schema check skipped: %s', exc)
            return applied

        for table, columns in ADDITIVE_COLUMNS.items():
            if table not in existing_tables:
                continue  # create_all() will build it with everything present
            present = {c['name'] for c in inspector.get_columns(table)}
            for column, ddl in columns.items():
                if column in present:
                    continue
                statement = f'ALTER TABLE {table} ADD COLUMN {column} {ddl}'
                try:
                    with db.engine.begin() as conn:
                        conn.execute(text(statement))
                    applied.append(f'{table}.{column}')
                    app.logger.info('Schema upgrade: added %s.%s', table, column)
                except Exception as exc:
                    app.logger.error('Schema upgrade failed for %s.%s: %s',
                                     table, column, exc)

    return applied