Question

i have users and entities (many-to-many) , and using sqlalchemy, with this model:

from sqlalchemy import Table, Column, Unicode, Integer, ForeignKey
from sqlalchemy.orm import relationship
from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy.orm import scoped_session, sessionmaker

from zope.sqlalchemy import ZopeTransactionExtension

DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))
Base = declarative_base()
users_entities = Table('users_entities', Base.metadata,
    Column('userID', Integer, ForeignKey('users.id')),
    Column('entitieID', Integer, ForeignKey('entities.id'))
)


class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    username = Column(Unicode(20))
    password = Column(Unicode(101))
    entities = relationship("Entities", secondary=users_entities)

    def __init__(self, username, password):
        self.username = username
        self.password = password


class Entities(Base):
    __tablename__ = 'entities'
    id = Column(Integer, primary_key=True)
    name = Column(Unicode(20))
    descr = Column(Unicode(101))
    url = Column(Unicode(101))

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

so, when i use:

user = dbsession.query(User).filter_by(id=session["userID"]).first()
entities = user.entities

i get the user with the user data, and the entities(user.entities) with all the user’s entities.

but instead of getting all the entities, now i need to get an entity from the user where the id is = X
something that would work like this:

user.query(Entities).filter_by(id=X)

i can’t find a simple(“best”) way to do this, am i missing something?

Answer 1

You are looking for Dynamic Relationship Loaders, they’re exactly what you want to do, and enable you to write:

user.entities.filter_by(id=X)

By the way, you can write user = dbsession.query(User).get(session["userID"]). It does the same query, but only if the object is not already in the session cache (plus, it’s shorter).