Question

Background Schema:

class Checkpoint(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    creator = db.Column(db.Integer, db.ForeignKey('user.id'))
    name = db.Column(db.String(255))
    description = db.Column(db.String(255), nullable=True)
    price = db.Column(db.Float, nullable=True)
    expiry = db.Column(db.DateTime, nullable=True)
    date_created = db.Column(db.DateTime)
    type = db.Column(db.String(255))
    image = db.Column(db.String(255))
    longitude = db.Column(db.Float)
    latitude = db.Column(db.Float)

class UserCheckpoint(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'))
    user = db.relationship("User")
    checkpoint_id = db.Column(db.Integer, db.ForeignKey('checkpoint.id'))
    checkpoint = db.relationship("Checkpoint")

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    email = db.Column(db.String(255))
    facebook_info = db.Column(db.String(255), db.ForeignKey('facebook_user.id'))
    facebook_user = db.relationship("FacebookUser")

class FriendConnection(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    fb_user_from = db.Column(db.String(255), db.ForeignKey('facebook_user.id'))
    fb_user_to = db.Column(db.String(255), db.ForeignKey('facebook_user.id'))

class FacebookUser(db.Model):
    id = db.Column(db.String(255), primary_key=True)
    name = db.Column(db.String(255))
    first_name = db.Column(db.String(255), nullable=True)
    middle_name = db.Column(db.String(255), nullable=True)
    last_name = db.Column(db.String(255), nullable=True)
    gender = db.Column(db.String(255), nullable=True)
    username = db.Column(db.String(255), nullable=True)
    link = db.Column(db.String(255), nullable=True)

I have a user, and as you can see, each user has a Facebook profile, as well as a table depicting inter-facebook-profile friendships. So given the user, the user would have a list of Facebook friends. I would like to get all UserCheckpoints that belong either to the user or his friends, with a given Checkpoint condition:

coord_conditions = and_(Checkpoint.longitude <= longitude + exp_no,
                                Checkpoint.longitude >= longitude - exp_no,
                                Checkpoint.latitude <= latitude + exp_no,
                                Checkpoint.latitude >= latitude - exp_no,
                                )

How can I do this using the ORM from SQLAlchemy? Thanks!

Summary: How to select UserCheckpoints given that the user_id belong to a list of friends/self; while UserCheckpoint.checkpoint has a set of conditions to fulfill.

Answer 1

Each relation has two methods to defined conditions on related objects: .has() for single referred object and .any() for collections. These methods allow straightforward translation of your task to SQLAlchemy expression. Let’s add missing relations to FacebookUser:

class FacebookUser(Model):
    # Definitions from question are here
    user = relationship(User, uselist=False)
    friends = relationship('FacebookUser',
                           secondary=FriendConnection.__table__,
                           primaryjoin=(id==FriendConnection.fb_user_from),
                           secondaryjoin=(FriendConnection.fb_user_to==id))

I’ve defined FacebookUser.user assuming one-to-one relation (which is usually supplemented with unique constraint on the foreign key column). Just remove uselist=False and adjust name if you allow several users being connected to one facebook account.

A shorter definition of your condition for coordinates:

coord_conditions = Checkpoint.longitude.between(longitude - exp_no,
                                                longitude + exp_no) & \
                   Checkpoint.latitude.between(latitude - exp_no,
                                               latitude + exp_no)

This condition is definitely wrong even for approximation (-179.9° and 179.9° are very close, while the difference is huge), but this is not main topic of the question.

A condition for users of interest (user with id equal to user_id and his friends):

user_cond = (User.id==user_id) | \
            User.facebook_user.has(
                FacebookUser.friends.any(FacebookUser.user.has(id=user_id)))

Now the query is quite simple:

session.query(UserCheckpoint).filter(
        UserCheckpoint.checkpoint.has(coord_conditions) & \
        UserCheckpoint.user.has(user_cond))

Unless you have (or expect) performance issues, I’d suggest avoid optimizing it at the cost of readability.