models.py

from sqlalchemy.ext.declarative import declarative_base
import sqlalchemy as db
from sqlalchemy.orm import relationship
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker
from datetime import datetime

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True)
    password = db.Column(db.String(12), unique=False)
    all_conversations = relationship('Conversation',
        primaryjoin='or_(User.id == Conversation.user1_id, User.id == ' \
        'Conversation.user2_id)', lazy='dynamic')
    # Replies where this is this user.
    replies = relationship("Reply", backref="user", lazy='dynamic')

class Conversation(Base):
    __tablename__ = 'conversation'
    id = db.Column(db.Integer, primary_key=True)
    user1_id = db.Column(db.Integer, db.ForeignKey("user.id"))
    user2_id = db.Column(db.Integer, db.ForeignKey("user.id"))        
    user1 = relationship('User', foreign_keys=[user1_id], 
        backref='conversations_as_user1')       
    user2 = relationship('User', foreign_keys=[user2_id], 
        backref='conversations_as_user2')
    # Replies to this conversation.
    replies = relationship("Reply", backref="conversation", lazy='dynamic')


class Reply(Base):
    __tablename__ = 'reply'
    id = db.Column(db.Integer, primary_key = True)
    reply = db.Column(db.Text)
    user_id = db.Column(db.Integer, db.ForeignKey("user.id"))
    conversation_id = db.Column(db.Integer, db.ForeignKey("conversation.id"))


engine = create_engine('sqlite://')


Session = scoped_session(sessionmaker(autoflush=False, autocommit=False))
Session.configure(bind=engine)

Base.metadata.create_all(engine)


user1 = User(username="user1", password="bla")
user2 = User(username="user2", password="bla")
Session.add(user1)
Session.add(user2)

print user1.username
print user2.username
c1 = Conversation(user1=user1, user2=user2)
Session.add(c1)

r1 = Reply(reply="yo yo", user=user2, conversation=c1)
Session.add(r1)
Session.commit()