from typing import TYPE_CHECKING
from sqlalchemy import Column, DateTime, String, ForeignKey, UniqueConstraint
from sqlalchemy.sql import func
from .base import AutoID, Base

# 只有在静态检查时才导入
# 运行阶段这段代码会被跳过，彻底避免循环导入
# 不确定是否用得上，但感觉这是个好习惯，尤其在模型之间有外键关系时
if TYPE_CHECKING:
    from .user import User

class Follow(Base, AutoID):
    """用户关注关系表"""
    __tablename__ = "follows"

    follower_id = Column(
        String(User.id_length()), 
        ForeignKey("users.id", ondelete="CASCADE"), 
        index=True
    )
    followee_id = Column(
        String(User.id_length()), 
        ForeignKey("users.id", ondelete="CASCADE"), 
        index=True
    )
    
    created_at = Column(DateTime(timezone=True), server_default=func.now())

    # 联合唯一索引：防止重复关注
    __table_args__ = (
        UniqueConstraint("follower_id", "followee_id", name="uq_follower_followee"),
    )