Flask-SqlAlchemy允许重复的多对多关系

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

student_course = db.Table('student_course',
    db.Column('student_id', db.Integer, db.ForeignKey('student.id')),
    db.Column('course_id', db.Integer, db.ForeignKey('course.id'))
)

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    courses = db.relationship('Course', secondary=student_course, backref=db.backref('students', lazy='dynamic'))

class Course(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))

student = Student.query.get(student_id)
course = Course.query.get(course_id)
student.courses.append(course)
db.session.commit()