"""单条消息（仅存 public 可见内容，隔离内部信息）。"""
from datetime import datetime
from sqlalchemy import String, Text, DateTime, ForeignKey
from sqlalchemy.orm import Mapped, mapped_column, relationship

from app.models.base import Base


class Message(Base):
    __tablename__ = "messages"

    id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
    session_id: Mapped[int] = mapped_column(ForeignKey("chat_sessions.id", ondelete="CASCADE"), nullable=False, index=True)
    role: Mapped[str] = mapped_column(String(16), nullable=False)  # user / assistant / system
    content: Mapped[str] = mapped_column(Text, nullable=False)
    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=datetime.utcnow)

    session: Mapped["ChatSession"] = relationship("ChatSession", back_populates="messages")