"""
Database engine/session management.

Uses SQLAlchemy so the storage backend is a config change, not a code
change. Today DB_URL points at a local SQLite file. When you're ready
to move to the MariaDB LXC, set DB_URL in .env to something like:

    mysql+pymysql://user:password@192.168.1.50:3306/order_manager

and install PyMySQL (pip install pymysql). Nothing else in the app
needs to change.
"""
from __future__ import annotations

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker, Session

from app import config
from app.models import Base

_engine = None
_SessionLocal = None


def get_engine():
    global _engine
    if _engine is None:
        db_url = config.get("DB_URL", config.DEFAULT_DB_URL)
        connect_args = {}
        if db_url.startswith("sqlite"):
            # allow use across the QThread worker and the UI thread
            connect_args = {"check_same_thread": False}
        _engine = create_engine(db_url, connect_args=connect_args, future=True)
    return _engine


def get_session_factory():
    global _SessionLocal
    if _SessionLocal is None:
        _SessionLocal = sessionmaker(bind=get_engine(), future=True, expire_on_commit=False)
    return _SessionLocal


def init_db() -> None:
    """Create tables that don't exist yet. Safe to call every startup."""
    Base.metadata.create_all(get_engine())


def get_session() -> Session:
    return get_session_factory()()