Question

I’m developing a small database where there are far more People than Users, so currently have the following Model.

class Person(db.Model):
  __tablename__ = 'people'
  id       = db.Column(db.Integer, primary_key = True)
  forename = db.Column(db.String(64))
  surname  = db.Column(db.String(64))

  memberships = db.relationship('Membership', backref='person')

  @property
  def name(self):
    return self.forename + ' ' + self.surname

  def __repr__(self):
    return '<Person %r %r>' % (self.forename, self.surname)

class User(Person):
  __tablename__ = 'users'
  id       = db.Column(db.Integer, db.ForeignKey('people.id'), primary_key = True)
  email    = db.Column(db.String(120), index = True, unique = True)
  role     = db.Column(db.SmallInteger, default = ROLE_USER)

  salt     = db.Column(db.BINARY(8))
  password = db.Column(db.BINARY(20))

  def __repr__(self):
    return '<User %r>' % (self.email)

It’s working quite well, in that if I create a User then a Person also get’s saved. The problem is creating a User when a Person already exists in the database.

I have tried the following:

>>> p = models.Person.query.get(3)
>>> u = models.User(id=p.id, email="[e-mail]")
>>> u.set_password('password')
>>> db.session.add(u)
>>> db.session.commit()
Traceback
...
sqlalchemy.exc.IntegrityError: (IntegrityError) PRIMARY KEY must be unique u'INSERT INTO people (id, forename, surname) VALUES (?, ?, ?)' (3, None, None)

What am I doing wrong? Is there a way to create a User from a Person?

Answer 1

On Michael Bayer’s advice, I’ve changed the polymorphism design and used composition instead. The code reads as follows…

class Person(db.Model):
  __tablename__ = 'people'
  id       = db.Column(db.Integer, primary_key = True)
  forename = db.Column(db.String(64))
  surname  = db.Column(db.String(64))

  memberships = db.relationship('Membership', backref='person')

  @property
  def name(self):
    return self.forename + ' ' + self.surname

  def __repr__(self):
    return '<Person %r %r>' % (self.forename, self.surname)

class User(db.Model):
  __tablename__ = 'users'
  id       = db.Column(db.Integer, db.ForeignKey('people.id'), primary_key = True)
  email    = db.Column(db.String(120), index = True, unique = True)
  role     = db.Column(db.SmallInteger, default = ROLE_USER)

  salt     = db.Column(db.BINARY(8))
  password = db.Column(db.BINARY(20))

  person   = db.relationship('Person')

A similar session to the one in the question would read…

>>> p = models.Person.query.get(3)
>>> u = models.User(person=p, email="[e-mail]")
>>> u.set_password('password')
>>> db.session.add(u)
>>> db.session.commit()
>>>