sqlalchemy relation through another (declarative)

Question

Is anyone familiar with ActiveRecord's "has_many :through" relations for models? I'm not really a Rails guy, but that's basically what I'm trying to do.

As a contrived example consider Projects, Programmers, and Assignments:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

from sqlalchemy import Column, ForeignKey
from sqlalchemy.types import Integer, String, Text
from sqlalchemy.orm import relation

from sqlalchemy.ext.declarative import declarative_base
Base = declarative_base()

class Assignment(Base):
    __tablename__ = 'assignment'
    id = Column(Integer, primary_key=True)
    description = Column(Text)

    programmer_id = Column(Integer, ForeignKey('programmer.id'))
    project_id = Column(Integer, ForeignKey('project.id'))

    def __init__(self, description=description):
        self.description = description

    def __repr__(self):
        return '<Assignment("%s")>' % self.description

class Programmer(Base):
    __tablename__ = 'programmer'
    id = Column(Integer, primary_key=True)
    name = Column(String(64))

    assignments = relation("Assignment", backref='programmer')

    def __init__(self, name=name):
        self.name = name

    def __repr__(self):
        return '<Programmer("%s")>' % self.name

class Project(Base):
    __tablename__ = 'project'
    id = Column(Integer, primary_key=True)
    name = Column(String(64))
    description = Column(Text)

    assignments = relation("Assignment", backref='project')

    def __init__(self, name=name, description=description):
        self.name = name
        self.description = description

    def __repr__(self):
        return '<Project("%s", "%s...")>' % (self.name, self.description[:10])

engine = create_engine('sqlite://')
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

Projects have many Assignments.

Programmers have many Assignments. (understatement?)

But in my office at least, Programmers also have many Projects - I'd like this relationship to be inferred through the Assignments assigned to the Programmer.

I'd like the Programmer model to have a attribute "projects" which will return a list of Projects associated to the Programmer through the Assignment model.

me = session.query(Programmer).filter_by(name='clay').one()
projects = session.query(Project).\
    join(Project.assignments).\
    join(Assignment.programmer).\
    filter(Programmer.id==me.id).all()

How can I describe this relationship clearly and simply using the sqlalchemy declarative syntax?

Thanks!

Answer 1

There are two ways I see:

1. Define a relation Programmer.projects with secondary='assignment'.

2. I define Assignment.project as relation and Programmer.projects as association_proxy('assignments', 'project') (probably you'd also like to define a creator). See Simplifying Association Object Relationships chapter for more information.