Non-circular recursive json printer. It's naive since I am sure there are more type cases that it can't handle but it's good enough for the most cases and trivial to extend.

BaseModel.py

class ModelBase(object):

    @declared_attr
    def __tablename__(cls):
        return cls.__name__.lower()

    def get_fields(self, view):
        """Retrieves the fields list defined for the specified view.
        """
        if view == 'list_view':
            fields = self.list_view
        elif view == 'single_view':
            fields = self.single_view
        else:
            fields = self.__mapper__.columns

        return fields


    def to_json(self, view='list_view', this_class=None):
        out = {}
        for f in self.get_fields(view):
            obj = getattr(self, f)
            if hasattr(obj, 'to_json'):
                if not this_class == self.__class__:
                    out[f] = obj.to_json(view=view, this_class=this_class)
            elif hasattr(obj, 'isoformat'):          
                out[f] = obj.isoformat()   
            elif isinstance(obj, datetime.timedelta):
                out[f] = (datetime.datetime.min + obj).time().isoformat()
            elif isinstance(obj, (basestring, int, long, )): 
                out[f] = obj 
            elif obj is None:
                out[f] = obj



        return dict(out)

Base = declarative_base(cls=ModelBase)