sqlalchemy多对多查询

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# coding:utf-8 from sqlalchemy import create_engine, Column, String, Integer, ForeignKey, Table from sqlalchemy.ext.declarative import declarative_base from sqlalchemy.orm import sessionmaker, relationship   DB_URI = "mysql+pymysql://root:123123sdt@127.0.0.1:3306/xiachufang" engine = create_engine(DB_URI) Base = declarative_base(engine) session = sessionmaker(engine)()   # 创建一个多对多的关系(老师与学生的关系)需要创建一个中间表 # 创建一个中间表 teacher_classes = Table(     "teacher_classes",     Base.metadata,     Column("teacher_id", Integer, ForeignKey("teacher.id"), nullable=False, primary_key=True),     Column("classes_id", Integer, ForeignKey("classes.id"), nullable=False, primary_key=True) )     # 创建老师的映射 class Teacher(Base):     __tablename__ = "teacher"     id = Column(Integer, primary_key=True, autoincrement=True)     teacher_name = Column(String(100))     classes = relationship("Classes", secondary=teacher_classes)       def __repr__(self):         return "<Teacher id='%s' teacher_name='%s'>" % (self.id, self.teacher_name)     # 创建学生的映射 class Classes(Base):     __tablename__ = "classes"     id = Column(Integer, primary_key=True, autoincrement=True)     classes_name = Column(String(100))     teacher = relationship("Teacher", secondary=teacher_classes)       def __repr__(self):         return "<Classes id='%s' classes_name='%s'>" % (self.id, self.classes_name)   # # # 创建数据库 # Base.metadata.create_all()   # # 创建两个老师 # teacher1 = Teacher(teacher_name='admin') # teacher2 = Teacher(teacher_name='grunt') # teacher3 = Teacher(teacher_name='shuihen')   # # 创建两门课程 # classes1 = Classes(classes_name="java") # classes2 = Classes(classes_name="python") # # # 添加数据 # teacher1.classes = [classes1,classes2] # teacher2.classes = [classes1,classes2] # teacher3.classes = [classes1] # session.add(teacher1) # session.add(teacher2) # session.add(teacher3) # session.commit()   # # 查询下数据(根据老师查询课程) # # teacher = session.query(Teacher).first() # # print teacher.classes # # # 根据课程查询老师 # classes = session.query(Classes).get(1) # print classes.teacher # # # 根据老师查询课程 # teacher = session.query(Teacher).get(3) # print teacher.classes