解决SQLAlchemy关系映射错误：单一Base对象的重要性

# app/schemas/user.py
from sqlalchemy import Column, Integer, String
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

# 独立的Base实例 1
Base = declarative_base()

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True, index=True)
    first_name = Column(String, index=True)
    # ... 其他字段
    favorites = relationship('Favorite') # 这里会出错，因为'Favorite'在当前Base的上下文中不可见

# app/schemas/favorite.py
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

# 独立的Base实例 2
Base = declarative_base()

class Favorite(Base):
    __tablename__ = "favorites"
    id = Column(Integer, primary_key=True, index=True)
    id_user = Column(Integer, ForeignKey('users.id')) # 注意：这里可以直接引用表名
    id_article = Column(Integer, ForeignKey('articles.id'))
    # user = relationship('User') # 如果这里也用独立的Base，也会面临同样问题

# app/database.py
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

# 定义数据库URL (示例，实际应用中应从配置加载)
DATABASE_URL = "sqlite:///./sql_app.db"

# 创建唯一的Base实例
Base = declarative_base()

# 创建数据库引擎
engine = create_engine(DATABASE_URL, connect_args={"check_same_thread": False})

# 创建会话本地工厂
SessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

# 数据库会话依赖函数
def get_db():
    db = SessionLocal()
    try:
        yield db
    finally:
        db.close()

# app/schemas/user.py
from sqlalchemy import Column, Integer, String
from sqlalchemy.orm import relationship
from app.database import Base # 从共享模块导入Base

class User(Base):
    __tablename__ = "users"

    id = Column(Integer, primary_key=True, index=True)
    first_name = Column(String, index=True)
    last_name = Column(String, index=True)
    email = Column(String, index=True)
    password = Column(String, index=True)

    # 现在'Favorite'将在同一个Base的元数据中被找到
    # 建议使用 back_populates 来建立双向关系
    favorites = relationship('Favorite', back_populates='user') 

# app/schemas/article.py (假设存在Article模型)
from sqlalchemy import Column, Integer, String
from app.database import Base # 从共享模块导入Base
from sqlalchemy.orm import relationship

class Article(Base):
    __tablename__ = "articles"

    id = Column(Integer, primary_key=True, index=True)
    title = Column(String, index=True)
    content = Column(String)

    # 如果Favorite需要引用Article，也需要定义关系
    # favorites = relationship('Favorite', back_populates='article') # 假设Favorite也有article关系

# app/schemas/favorite.py
from sqlalchemy import Column, Integer, ForeignKey
from sqlalchemy.orm import relationship
from app.database import Base # 从共享模块导入Base
# 不需要导入User或Article模型类本身，relationship会通过字符串名称查找

class Favorite(Base):
    __tablename__ = "favorites"

    id = Column(Integer, primary_key=True, index=True)
    # 使用字符串引用表名，更健壮
    id_user = Column(Integer, ForeignKey('users.id')) 
    id_article = Column(Integer, ForeignKey('articles.id'))

    # 建立与User的双向关系
    user = relationship('User', back_populates='favorites')    
    # 建立与Article的双向关系（如果Article也定义了相应的back_populates）
    # article = relationship('Article', back_populates='favorites') 

# app/main.py (或您的表创建脚本)
from app.database import engine, Base # 导入共享的engine和Base

# 确保所有模型模块都被导入，以便它们的定义被Base.metadata收集
import app.schemas.user
import app.schemas.article # 假设存在
import app.schemas.favorite

# 使用统一的Base.metadata创建所有表
Base.metadata.create_all(bind=engine)
print('所有数据库表已创建！')

# 后续的SessionLocal和get_db函数已在app/database.py中定义并可直接使用
# from app.database import SessionLocal, get_db