SQLAlchemy Inheritance prevent of creating base instance

Question

I have created the following model:

class Person(db.Model):

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(1024), nullable=False)
    email = db.Column(db.String(250), nullable=True, unique=True)
    code = db.Column(db.String(10), nullable=False, unique=True)

There are some cases in which a Person turnos into a system user. So, I have the following model:

class User(Person):
    password = db.Column(db.String(1024), nullable=False)

The problem, I that when I try to create an User from a Person doing this:

>>> person = Person(name='foobar', email='[email protected]', code='123123')
>>> db.session.add(person)
>>> db.session.commit()
>>> print person.id
4
>>> user = User(id=4, password='changeme')
>>> db.session.add(user)
>>> db.session.commit()
(IntegrityError) null value in column "name" violates not-null constraint
'INSERT INTO person (id, name, ....

The question is how can I prevent of the model User creating a new Person instance because that already exists on the database?

UPDATE: when trying Join table inheritance, I have the same problem. It don't understand how not to create a new instance of Person

class Person(db.Model):

    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(1024), nullable=False)
    email = db.Column(db.String(250), nullable=True, unique=True)
    code = db.Column(db.String(10), nullable=False, unique=True)
    person_type = db.Column(db.String(10), nullable=False)


    __mapper_args__ = {'polymorphic_on': 'person_type'}

class User(Person):
    __mapper_args__ = {'polymorphic_identity': 'recurvado'}

    id = db.Column(db.Integer, db.ForeignKey('arco.id'), primary_key=True)
    password = db.Column(db.String(1024), nullable=False)

The problem, I that when I try to create an User from a Person doing this:

>>> person = Person(name='foobar', email='[email protected]', code='123123')
>>> db.session.add(person)
>>> db.session.commit()
>>> print person.id
4
>>> user = User(id=4, password='changeme')
>>> db.session.add(user)
>>> db.session.commit()
(IntegrityError) null value in column "name" violates not-null constraint
'INSERT INTO person (id, name, .

I think the error states the issue right away: You create a new User but you haven't specified a name, a code and whatever else has a NOT NULL constraint. From your example I understand that you just want to do this: user = User(name='foobar', email='foobar...', password='changeme'); session.add(user); session.commit() The Person class does not need to be involved. Also, I don't think you can have a non-null password when using joined inheritance, all your Persons will raise integrity errors too — dtheodor
– dtheodor, Commented Jul 29, 2014 at 20:01
The question that I have is: if I have already created a Person, who can I create an User from it without creating a new Person. — tzulberti
– tzulberti, Commented Jul 30, 2014 at 11:11
Haven't tried this upgrading before. I would try like this: user = session.query(User).get(4); user.password = '...' If not, perhaps this: person = session.query(Person).get(4); person.person_type = 'recurvado' ;session.commit() user = session.query(User).get(4) — dtheodor
– dtheodor, Commented Jul 31, 2014 at 9:41

dtheodor · Accepted Answer · 2014-07-28 12:15:05Z

1

There are a couple of ways. The simple one

class Person(object): #notice there's no db.Model here
    id = db.Column(db.Integer, primary_key=True)
    #the rest of the person columns follow

class User(Person):
    password = db.Column(db.String(1024), nullable=False)
    #other user specific attributes and columns

The above is good if you never want your Person class to be instantiated as a db object itself. If you actually want that, you should look into http://docs.sqlalchemy.org/en/rel_0_9/orm/inheritance.html#single-table-inheritance and pick what's best for you, the documentation is extensive.

answered Jul 28, 2014 at 12:15

dtheodor

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tzulberti Over a year ago

Updated the questing with some example

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