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()

What exactly are you trying to do? You do realize the way this is defined in a way that sqlalchemy expects a new table for UserWithExtraMethod, resulting in that error message because the __tablename__ is not defined for that. Tangentially relevant info: stackoverflow.com/a/1337871/2904896 — metatoaster
– metatoaster, Commented Apr 10, 2014 at 2:24
I am not trying to create extra table or do any inheritance. I just want to add an extra method after the entity is created. It want to extend the class without any side effect to the underlying SQL mapper. — hllau
– hllau, Commented Apr 10, 2014 at 3:16
I would like the different modules to have different implementations of the same method. — hllau
– hllau, Commented Apr 10, 2014 at 4:53
Which sqlalchemy version are you using? I am unable to actually reproduce your issue with either latest version of 0.9 or 0.8. Alternatively, it's possible your code example alone can't trigger this. — metatoaster
– metatoaster, Commented Apr 10, 2014 at 5:21
"Different implementations"? are you sure you want that? Are you giving your persistent classes too many responsibilities? what are some of the different implementations you will need? — SingleNegationElimination
– SingleNegationElimination, Commented Apr 11, 2014 at 21:02

SingleNegationElimination · Accepted Answer · 2014-04-11 20:57:00Z

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

Victor Uriarte · Accepted Answer · 2015-10-10 18:02:17Z

4

Perhaps a little late for this reply. Do you have any other relationships setup that are pointing to User?

For example, if you have Address defined as:

class Address(Base):
    __tablename__ = 'address'
    id = Column(Integer, primary_key=True)
    address = Column(String(50))
    Users = relationship("User", backref="addresses")

when Address is trying to resolve to which User within the declarative base to point to, it will find two of them. To verify try Base._decl_class_registry['User']. This is similar to this topic covered by Michael.

In ./sqlalchemy/ext/declarative/clsregistry.py there is an example on how to use the fully qualified path. In this case it would be changing the relationship within address from Users = relationship("User", backref="addresses") to Users = relationship("a.User", backref="addresses")

Hope this helps point you in the right direction for debugging.

answered Oct 10, 2015 at 18:02

Victor Uriarte

4894 silver badges9 bronze badges

1 Comment

Victor Uriarte Over a year ago

One thing I am curious is to check when the second User class is being defined. That may help also clear out some of the smoke.

van · Accepted Answer · 2014-04-10 08:27:07Z

1

Hacky, but why not just monkey-patch the User class for your purpose instead of inheriting from it?

# modude b
from a import User

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

User.name_capitalized = name_capitalized    
user = User() # and it has extra-method as well
print(user.name_capitalized)

answered Apr 10, 2014 at 8:27

van

77.6k13 gold badges174 silver badges179 bronze badges

1 Comment

hllau Over a year ago

I am currently doing this, just monkey patching isn't proper. The problem is SQLAlchemy declarative class has a metaclass that create the entity. There seems to be no way to disable invoking the metaclass from creating it again.

Akash · Accepted Answer · 2014-04-11 20:33:28Z

1

This may not work for you. I had a similar issue. I ended up passing an instance of User to UserWithExtraMethod during instantiation

class UserWithExtraMethod(object):
    def __init__(self, user):
        self.user = user

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

Hope this helps

answered Apr 11, 2014 at 20:33

Akash

3494 silver badges14 bronze badges

Collectives™ on Stack Overflow

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

4 Answers 4

Comments

1 Comment

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

1 Comment

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related