How do I set a foreign key in the constructor of a SQLAlchemy (Python, Flask) db model?

Question

For example, I have 2 models:, one of a student and one of a teacher.
Do I have to add to Student in the constructor self.teacher_id = teacher_id ?

class Student(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Text)
    age = db.Column(db.Integer)
    teacher_id = db.Column(db.Integer, db.ForeignKey('teacher.id'))
    def __init__(self, name, age):
        self.name = name
        self.age = age

class Teacher(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    subject = db.Column(db.Text)
    students = db.relationship('Student', backref='person', lazy='dynamic')
    def __init__(self, subject):
        self.subject = subject

Answer 1

Since one student has many teachers and one teacher has many students, its a many-to-many relationship and you need to add an association table to do that:

class Student(db.Model):
    __tablename__ = 'student'
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.Text)
    age = db.Column(db.Integer)
    teachers = db.relationship('Teacher', secondary='xtable', backref=db.backref('students', lazy='dynamic'))

class Teacher(db.Model):
    __tablename__ = 'teacher'
    id = db.Column(db.Integer, primary_key=True)
    subject = db.Column(db.Text)

class Xtable(db.Model):
    __tablename__ = 'xtable'
    student_id = db.Column('student_id', db.ForeignKey('student.id'))
    teacher_id = db.Column('teacher_id', db.ForeignKey('teacher.id'))

And now you should be able to query Student.teachers and Teacher.students

Docs: http://docs.sqlalchemy.org/en/latest/orm/basic_relationships.html#many-to-many

*Note that when using the backref parameter instead of relationship.back_populates, the backref will automatically use the same secondary argument for the reverse relationship. So you don't have to put the relationship statement in both classes