I'm new to SQLAlchemy and relational databases, and I'm trying to set up a model for an annotated lexicon. I want to support an arbitrary number of key-value annotations for the words which can be added or removed at runtime. Since there will be a lot of repetition in the names of the keys, I don't want to use this solution directly, although the code is similar.

My design has word objects and property objects. The words and properties are stored in separate tables with a property_values table that links the two. Here's the code:

from sqlalchemy import Column, Integer, String, Table, create_engine
from sqlalchemy import MetaData, ForeignKey
from sqlalchemy.orm import relation, mapper, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

engine = create_engine('sqlite:///test.db', echo=True)
meta = MetaData(bind=engine)

property_values = Table('property_values', meta,
    Column('word_id', Integer, ForeignKey('words.id')),
    Column('property_id', Integer, ForeignKey('properties.id')),
    Column('value', String(20))
)
words = Table('words', meta,
    Column('id', Integer, primary_key=True),
    Column('name', String(20)),
    Column('freq', Integer)
)
properties = Table('properties', meta,
    Column('id', Integer, primary_key=True),
    Column('name', String(20), nullable=False, unique=True)
)
meta.create_all()

class Word(object):
    def __init__(self, name, freq=1):
        self.name = name
        self.freq = freq

class Property(object):
    def __init__(self, name):
        self.name = name
mapper(Property, properties)  

Now I'd like to be able to do the following:

Session = sessionmaker(bind=engine)
s = Session()
word = Word('foo', 42)
word['bar'] = 'yes' # or word.bar = 'yes' ?
s.add(word)
s.commit()

Ideally this should add 1|foo|42 to the words table, add 1|bar to the properties table, and add 1|1|yes to the property_values table. However, I don't have the right mappings and relations in place to make this happen. I get the sense from reading the documentation at http://www.sqlalchemy.org/docs/05/mappers.html#association-pattern that I want to use an association proxy or something of that sort here, but the syntax is unclear to me. I experimented with this:

mapper(Word, words, properties={
    'properties': relation(Property, secondary=property_values)
    })

but this mapper only fills in the foreign key values, and I need to fill in the other value as well. Any assistance would be greatly appreciated.