SQL-Alchemy join query - most efficient way to query

Question

I am trying to generate a query and having difficulty finding the most efficient way to do it in sqlalchemy, (note I'm using flask-sqlalchemy)

The goal is to find all users have a meeting with a specific user.

So let's say Frank has 10 meetings coming up, I want to generate a list of all people frank has a meeting with.

Here are my models:

class UserMeeting(db.Model):
    """ Associative table, links meetings to users in a many to many fashion"""
    __tablename__ = 'userMeeting'
    id = db.Column(db.Integer, primary_key = True)
    meeting_id = db.Column(db.Integer, db.ForeignKey('meeting.id'), primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), primary_key=True)

class Meeting(db.Model):
    __tablename__ = "meeting"
    id = db.Column(db.Integer, primary_key = True)
    title = db.Column(db.String(128))
    #... other columns
    #associative reference
    attendees = db.relationship('UserMeeting', backref='meeting')

class User(db.Model):
    __tablename__ = 'user'
    id = db.Column(db.Integer, primary_key = True)
    email = db.Column(db.String(128), index=True, unique=True)
    password = db.Column(db.String(128))
    #associative reference
    attendingMeetings = db.relationship("UserMeeting", backref="user", cascade="all, delete-orphan") 

Here's what I've tried:

#Assume frank's a user with id == 1
frank = User.query.get(1)
franks_meetings = Meeting.query.join(Meeting.attendees).filter(UserMeeting.user == frank).all()
#not efficient way of getting users in meetings with frank
users = []
for meeting in franks_meetings:
    for userMeeting in meeting.attendees:
        if userMeeting.user != frank:
            users.append(userMeeting.user)

#is there a way to just generate one query and get this data?

I seem to be missing how I could just use joins to get this data. Any help would be appreciated!

Answer 1

You need to join the UserMeeting table with itself, using meeting_id as the join key. You may need to alias the table in order to reference it twice. I don't know if I can type the sqlalchemy syntax for it off the top of my head, but the sql looks like:

select distinct(b.user_id) as other_user_id
from usermeeting a
inner join usermeeting b
on a.meeting_id=b.meeting_id
where a.user_id=1 and b.user_id != 1;

And 1 is Frank.

Oh, and getting the user details too. Probably you could end up with User objects directly from doing this in sqlalchemy:

select distinct(u.id), u.email
from usermeeting a
inner join usermeeting b
on a.meeting_id=b.meeting_id
inner join users u
on b.user_id=u.id
where a.user_id=1 and b.user_id != 1;

Answer 2

Here's the sqlalchemy version of the query for reference:

#get all users in meetings with Frank, (frank.id == 1)
um = aliased(UserMeeting)
frank = User.query.get(1)

q = session.query(User).join(User.attendingMeetings).\
    filter(UserMeeting.meeting_id == um.meeting_id).\
    filter(UserMeeting.user_id != frank.id, um.user_id == frank.id)

users_meeting_with_frank = q.all()