Using non-database-backed models in SQLAlchemy

Question

I have an SQLAlchemy setup where one of my models is not stored in the database. The data comes off a web service instead and other models only store a primary key to refer to it.

How do I make this work while still using SQLAlchemy relationships? I'm caching the data locally in the database right now and updating from the web service when something changes, but this means I'm storing redundant copies of the data. I'd like to not store locally at all. Consider this:

Base = declarative_base()

# Web service-backed model
class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    username = Column(Unicode(20), nullable=False)

    @classmethod
    def get(cls, id):
        """Get an instance using data from the web service."""
        pass

# Local model
class Document(Base):
    __tablename__ = 'document'
    id = Column(Integer, primary_key=True)
    user_id = Column(Integer, ForeignKey('user.id'))
    user = relationship(User, backref='documents')
    content = Column(UnicodeText)

I'd like to remove the User model from my database and retrieve it only from the web service (which is straightforward), while still preserving the benefits of a bi-directional relationship as provided by SQLAlchemy (Document.user and User.documents).

How do I achieve the latter?

zzzeek · Accepted Answer · 2012-10-31 02:45:18Z

1

we haven't put much effort into this model though there have been some sketches of ideas put forth that could eventually lead to some models being persisted in non-relational store - but the effort here is more on the unit of work side. On the query/loading side, you could maybe achieve some of what you're looking for using some relatively new features of SQLA - in 0.7 there is the load_on_pending flag of relationship (http://docs.sqlalchemy.org/en/rel_0_7/orm/relationships.html#relationships-api) which would allow you to create a User object, give it the primary key you want, add() it to a Session and then without any flush the .documents collection should emit a load; in 0.8 there's the better solution enable_relationship_loading (http://docs.sqlalchemy.org/en/latest/orm/session.html#sqlalchemy.orm.session.Session.enable_relationship_loading) method, if you attach a User object using this method the object will stay detached from the Session, but still use it when you invoke user.documents.

These are both rough features I added for the benefit of just one user, not necessarily tailored to what you're doing here, but if you try them out let me know how it goes.

answered Oct 31, 2012 at 2:45

zzzeek

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1 Comment

Kiran Jonnalagadda Over a year ago

Thanks. I have some free time coming up in November and will poke at this.

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