Problem with sqlAlchemy model

So, I have the following situation. I have a DataTypes class that has the following structure:

class DataType(Base):
    __tablename__ = 'DATA_TYPES'
    id = Column(Integer, primary_key=True)
    type_name = Column(String)
    fk_result_storage = Column(Integer, ForeignKey('DATA_STORAGES.id'))

    parentDataStorage = relationship("DataStorage", backref=backref("DataType",    cascade="all,delete"))

    def __init__(self, name, resultId):
       self.type_name = name
       self.fk_result_storage = resultId

Now the relationships defined here are working. But now I have some specific data types that are created dynamically through introspection and which also need to be excluded. They are created as follows:

t = Table('DATA_' + obj.__name__.lower(), *t[:-1], **t[-1])
mapper(obj, t, *args, **kwargs)
model.Base.metadata.create_all(dao.Engine)   

This works great, and tables are created as needed. But now I want to add relationships in the same way as from the DataType class. So I tried this:

t = T('DATA_' + obj.__name__.lower(), *t[:-1], **t[-1])
M(obj,t,properties = {'children' : relationship('DataType', backref=backref(obj, cascade="all,delete"))} )   
model.Base.metadata.create_all(dao.Engine)

But it gives me:

 sqlalchemy.exc.InvalidRequestError: One or more mappers failed to initialize - can't proceed with initialization of other mappers.  Original exception was: relationship 'children' expects a class or a mapper argument (received: <type 'str'>)

I am very new to SQLAlchemy. Any suggestions?

Regards, Bogdan