How do I extend a SQLAlchemy bound declarative model with extra methods?

Question

For example, I have a declarative class on module a:

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    addresses = relationship("Address", backref="user")

Now, in module b I want to use the mapped entity, but add a method:

from a import User

class UserWithExtraMethod(User):
    def name_capitalized(self):
        return self.name.capitalize()

user = UserWithExtraMethod()
print(user.name_capitalized)

However, when I run the script, I will get the following error:

InvalidRequestError: Multiple classes found for path "User" in the registry of this declarative base. Please use a fully module-qualified path.

What have I missed when declaring the user entity? I would like to reuse the previous declared entity.

I am expecting something would be like:

class UserWithExtraMethod(User):
    ___magic_reuse_previous_mapper__ = True

    def name_capitalized(self):
        return self.name.capitalize()

Answer 1

Unless you've got a particular reason to have separate classes, you should just write:

class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    addresses = relationship("Address", backref="user")

    def name_capitalized(self):
        return self.name.capitalize()

Since the name_capitalized is not special as far as SQLAlchemy is concerned (it's not a ColumnExpression or some such), it is completely ignored by the mapper.

Actually, there's an even better way to do this; your version works fine for instances of User, but is of no use in sql expressions.

from sqlalchemy.ext.hybrid import hybrid_property, hybrid_method
class User(Base):
    # ... body as before

    @hybrid_method
    def name_capitalized(self):
        return self.name.capitalize()

    @name_capitalized.expression
    def name_capitalized(cls):
        # works for postgresql, other databases spell this differently.
        return sqlalchemy.func.initcap(cls.name)

which will allow you to do things like:

>>> print Query(User).filter(User.name_capitalized() == "Alice")
SELECT users.id AS users_id, users.name AS users_name 
FROM users 
WHERE initcap(users.name) = :initcap_1