Need some help accessing a ForeignKey row's data while performing a select with SQLAlchemy - sql

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.

Answer 2

Basically your query can be split in two parts:
Given the user_id, create a list of users which will contain the user herself as well as all direct friends
Given the list of users from 1., get all UserCheckpoint whose Checkpoint would satisfy the criteria.
Not tested code:
# get direct user for given user_id
u1 = (session.query(User.id.label("user_1_id"), User.id.label("user_id"))
)
# get friends of the user in one direction (from other user to this one)
User2 = aliased(User)
FacebookUser2 = aliased(FacebookUser)
u2 = (session.query(User2.id.label("user_1_id"), User.id.label("user_id")).
join(FacebookUser2, User2.facebook_info == FacebookUser2.id).
join(FriendConnection, FacebookUser2.id == FriendConnection.fb_user_from).
join(FacebookUser, FacebookUser.id == FriendConnection.fb_user_to).
join(User, User.facebook_info == FacebookUser.id)
)
# get friends of the user in other direction (from this user to the other)
User2 = aliased(User)
FacebookUser2 = aliased(FacebookUser)
u3 = (session.query(User2.id.label("user_1_id"), User.id.label("user_id")).
join(FacebookUser2, User2.facebook_info == FacebookUser2.id).
join(FriendConnection, FacebookUser2.id == FriendConnection.fb_user_to).
join(FacebookUser, FacebookUser.id == FriendConnection.fb_user_from).
join(User, User.facebook_info == FacebookUser.id)
)
# create a union to have all pairs (me_or_friend_id, user_id)
u_all = union_all(u1, u2, u3)
# **edit-1: added alias **
u_all = u_all.alias("user_list_view")
# final query which adds filters requirested (by user_id and the checkpoint condition)
q = (session.query(UserCheckpoint).
join(Checkpoint).filter(coord_conditions).
join(u_all, UserCheckpoint.user_id == u_all.c.user_1_id).
filter(u_all.c.user_id == user_id)
)
for u_cp in q.all():
print u_cp
Note, that you could simplify the query somewhat if you defined more relationships in your model and then can remove some primaryjoin conditions from join clauses.