首个可运行的版本

backend/app/models/friend.py

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+"""好友关系模型"""
+
+from datetime import datetime, timezone
+
+from sqlalchemy import String, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.database import Base
+
+
+class Friend(Base):
+    __tablename__ = "friends"
+    __table_args__ = (
+        UniqueConstraint("user_id", "friend_user_id", name="uq_friendship"),
+    )
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True)
+    user_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    friend_user_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    remark: Mapped[str | None] = mapped_column(String(50), nullable=True)  # 好友备注
+    created_at: Mapped[datetime] = mapped_column(DateTime, default=lambda: datetime.utcnow())
+
+    # 关系
+    user = relationship("User", foreign_keys=[user_id], back_populates="friends")
+    friend_user = relationship("User", foreign_keys=[friend_user_id], back_populates="friend_of")