Class Parent(db.Model):
  id = db.Column(db.Integer, nullable=False, autoincrement=True, primary_key=True)
  name = db.Column(db.String, nullable=False)

  children = relationship('Child',
                                secondary=parent_has_children,
                                back_populates='parents'
                                )


Class Child(db.Model):
  id = db.Column(db.Integer, nullable=False, autoincrement=True, primary_key=True)
  name = db.Column(db.String, nullable=False)

  parents = relationship('Parent',
                                secondary=parent_has_children,
                                back_populates='children'
                                )

parent_has_children = db.Table('parent_has_children', db.metadata,
                            db.Column('parent_id', db.Integer, ForeignKey('Parent.id')),
                            db.Column('child_id', db.Integer, ForeignKey('Child.id'))
                            )

@app.route('/buildrelationship', methods=['POST'])
def buildrelationship():
 child_id= request.json['student_id']
 parent_id = request.json['parent_id']
 child = Child.query.get(child_id)
 parent = Parent.query.get(parent_id)
 parent.children.append(child)
 db.session.commit()