JSON encoder for SQLAlchemy models, using the inspection API. Not thoroughly tested. No warranty, blah blah. Improvements welcomed!

modeljson.py

import json
from sqlalchemy.inspection import inspect

class AlchemyEncoder(json.JSONEncoder):
    '''JSON encoder for SQLAlchemy models, using inspection API'''

    def __init__(self, skipkeys=False, ensure_ascii=True,
                 check_circular=True, allow_nan=True, sort_keys=False,
                 indent=None, separators=None, default=None, relations=False,
                 **kw):

        self._include_relations = relations
        super(AlchemyEncoder, self).__init__(
                skipkeys=False,
                ensure_ascii=True,
                check_circular=True,
                allow_nan=True,
                indent=None,
                separators=None,
                default=None,
                **kw
            )

    def default(self, obj):
        ''' 
        inspect columns of sqlalchemy model, create secondary encoder if
        necessary for relationships and make it so we don't go more than one
        level deep.
        '''
        info = inspect(obj)
        if not info.is_instance:
            raise TypeError('%s is not an instance' % obj)

        output = {}

        cols = info.mapper.columns
        for colname in cols.keys():
            if colname in info.dict:
                output[colname] = info.dict[colname]
            else:
                output[colname] = None

        if self._include_relations:
            encoder = AlchemyEncoder(relations=False)
            for relname, relinfo in info.mapper.relationships.items():
                if relname in info.dict:
                    if relinfo.uselist:
                        output[relname] = []
                        for relobj in info.dict[relname]:
                            if relobj == obj:
                                output[relname].append({'self': True})
                            else:
                                output[relname].append(encoder.default(relobj))
                    else:
                        output[relname] = encoder.default(relobj)

        return output

def model_json(obj, relations=False):
    return json.dumps(obj, cls=AlchemyEncoder, relations=relations)