---

from sqlalchemy import Column, Integer, String, ForeignKey
from sqlalchemy.orm import declarative_base, relationship

Base = declarative_base()

class Student(Base):
    __tablename__ = 'students'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    
    grades = relationship("Grade", back_populates="student")

class Teacher(Base):
    __tablename__ = 'teachers'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    
    grades = relationship("Grade", back_populates="teacher")

class Subject(Base):
    __tablename__ = 'subjects'

    id = Column(Integer, primary_key=True)
    name = Column(String, nullable=False)
    
    grades = relationship("Grade", back_populates="subject")

class Grade(Base):
    __tablename__ = 'grades'

    id = Column(Integer, primary_key=True)
    value = Column(Integer, nullable=False)
    
    student_id = Column(Integer, ForeignKey('students.id'), nullable=False)
    teacher_id = Column(Integer, ForeignKey('teachers.id'), nullable=False)
    subject_id = Column(Integer, ForeignKey('subjects.id'), nullable=False)

    student = relationship("Student", back_populates="grades")
    teacher = relationship("Teacher", back_populates="grades")
    subject = relationship("Subject", back_populates="grades")