python inheritance in sqlalchemy

Question

So I'm new to this python and sqlalchemy. I need some help with inheritance or maybe a mixin (but rather inheritance).

I have some psudo code but I haven't really made any progress to get anywhere:

Base = declarative_base()

class ModelBase(Base):
  """Base model that only defines last_updated"""
  __tablename__ = 'doesnotexistandtheclassshouldnotbeinstantiated'

  #all tables inheriting from ModelBase will have this column
  last_updated = Column(DateTime)

  def __init__(self, last_updated):
    self.last_updated = last_updated

class User(ModelBase):
  """Defines the user but should also have the last_updated inherited from ModelBase"""
  __tablename__ = 'user'

  id = Column(Integer, primary_key=True)

  def __init__(self, ....):
    ModelBase.__init__(last_updated)

I want all tables inheriting from ModelBase to also have last_updated. How would I do that?

UPDATED CODE:

class BaseUserMixin(object):
    """Base mixin for models using stamped data"""

    @declared_attr
    def last_updated(cls):
        return Column(DateTime)

    @declared_attr        
    def last_updated_by(cls):
        return Column(String)

    def __init__(self, last_updated, last_updated_by):
        self.last_updated = last_updated
        self.last_updated_by = last_updated_by

Base = declarative_base(cls=BaseUserMixin)


class User(Base):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    email = Column(String)
    fullname = Column(String)
    password = Column(String)
    enabled = Column(Boolean)

    def __init__(self, name, fullname, password, email, last_updated, last_updated_by):
        self.name = name
        self.fullname = fullname
        self.password = password
        self.email = email
        # goes wrong here
        super(User, self).__init__(last_updated, last_updated_by)

    def __repr__(self):
        return "<User('%', '%', '%', '%', '%', '%')>"\
               % (self.name,
                  self.fullname,
                  self.password,
                  self.email,
                  self.last_updated,
                  self.last_updated_by
                  )

The error is:

_declarative_constructor() takes exactly 1 argument (3 given)

What can be the problem? I thought it was working but when re-running the debugger it failed.

Did you see this question? stackoverflow.com/questions/1337095/sqlalchemy-inheritance That seems to be a reasonably good answer. — Magsol
– Magsol, Commented Mar 20, 2013 at 20:37
yes. the thing is that they are not related and it's not 'polymorphic_on' anything... — Asken
– Asken, Commented Mar 20, 2013 at 20:41

SingleNegationElimination · Accepted Answer · 2013-03-21 15:41:06Z

The solution is declared_attr; which will be called and added to instances of DeclarativeMeta anytime they appear:

Edit: the __init__ automagically provided by declarative cannot call super(). if you want it, it has to be last, and the only way to do that is to use a regular mixin.

import datetime
from sqlalchemy import Column, DateTime, Integer, String
from sqlalchemy.ext.declarative import declared_attr, declarative_base

class BaseMixin(object):
    @declared_attr
    def last_updated(cls):
        return Column(DateTime)

    def __init__(self, last_updated, *args, **kwargs):
        super(BaseMixin, self).__init__(last_updated=datetime.datetime.now(), *args, **kwargs)
        print "BaseMixin.__init__"
        self.last_updated = last_updated

ModelBase = declarative_base()

Note that the mixin must come first!

class User(BaseMixin, ModelBase):
    """Defines the user but should also have the last_updated inherited from ModelBase"""
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    username = Column(String)

    def __init__(self, *args, **kwargs):
        super(User, self).__init__(last_updated=datetime.datetime.now(), *args, **kwargs)
        print "User.__init__"

if __name__ == '__main__':
    from sqlalchemy import create_engine
    from sqlalchemy.orm import sessionmaker
    engine = create_engine('sqlite:///:memory:', echo=True)
    ModelBase.metadata.create_all(engine)
    user = User(username='alice')

    Session = sessionmaker(engine)
    session = Session()
    session.add(user)
    session.commit()

However; Are you sure you want to use __init__ for this in the first place? __init__ is not called when objects are returned from queries; and what you really want is for the column to change to right now when it's modified. That's baked into Column() already:

from sqlalchemy import func

class BaseMixin(object):
    @declared_attr
    def created_date(cls):
        return Column(DateTime, default=func.now())

    @declared_attr
    def modified_date(cls):
        return Column(DateTime, default=func.now(), onupdate=func.now())

back to using the cls= argument

ModelBase = declarative_base(cls=BaseMixin)


class User(ModelBase):
    __tablename__ = 'user'

    id = Column(Integer, primary_key=True)
    username = Column(String)

if __name__ == '__main__':
    engine = create_engine('sqlite:///:memory:', echo=True)
    ModelBase.metadata.create_all(engine)
    user = User(username='alice')

    Session = sessionmaker(engine)
    session = Session()
    session.add(user)
    session.commit()

    session = Session()
    sameuser = session.query(User).one()
    sameuser.username = 'bob'
    session.commit()

super(User, self).__init__ will call ALL init functions. is that the problem?

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