CODEWITHSUNDEEP
pythonormsqlalchemy
Sam Hosseini
Sam Hosseini

Reputation: 757

SQLAlchemy: exclude rows taken from a subquery on a query

Abstraction of my problem, I have 2 tables. A User table, and a Friendship table.

I'm trying to make a query to list all the users available to be added as friend to User 1, Alice, and also excluding herself, using SQLAlchemy.

Considering there could be a lot of friendships, to find Alice's friends: 

friend_subquery = db.session.query(Friendship).filter_by(User_id=1).subquery()

Now I want all the users listed, except Alice, and her friends, Bob and Jack. 

friends = (db.session.query(User).
            filter(User.ID != 1).
            outerjoin(friend_subquery,
                      User.ID != friend_subquery.c.Friend_id))

My expected result would have been to get User 4 and 5, but this query returns all except Alice herself. The condition of 

User.ID != friend_subquery.c.Friend_id

seem NOT to be working as expected.

P.S. I've done my homework of searching, reading docs, but couldn't figure it out. Thanks for your time.

Upvotes: 6

Views: 14123

Answers (1)

van
van

Reputation: 77092

I assumed that your models are defined as below:

class User(db.Model):
    __tablename__ = 'User'

    ID = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100))

    friendships = db.relationship(
        'Friendship',
        foreign_keys='Friendship.User_id',
        backref='friender',
    )
    friendships_of = db.relationship(
        'Friendship',
        foreign_keys='Friendship.Friend_id',
        backref='friendee',
    )


class Friendship(db.Model):
    __tablename__ = 'Friendship'
    ID = db.Column(db.Integer, primary_key=True)
    User_id = db.Column(db.Integer, db.ForeignKey('User.ID'))
    Friend_id = db.Column(db.Integer, db.ForeignKey('User.ID'))

In which case two ways to perform this query is shown in the code below. The first query relies on the relationship User.friendships_of, while the second works with explicit joins:

    # Add users
    u1, u2, u3, u4, u5 = users = [
        User(name="Alice"),
        User(name="Bob"),
        User(name="Jack"),
        User(name="Pluto"),
        User(name="Mike"),
    ]
    db.session.add_all(users)

    # Add friendhips
    u1.friendships.append(Friendship(friendee=u2))
    u1.friendships.append(Friendship(friendee=u3))
    db.session.commit()

    # Find Alice
    u_alice = db.session.query(User).filter(User.name == 'Alice').one()

    # Query (version 1)
    q = (
        db.session.query(User)
        .filter(~User.friendships_of.any(Friendship.User_id == u_alice.ID))
        .filter(User.ID != u_alice.ID)
        .all()
    )
    for x in q:
        print(x)

    # Query (version 2)
    q = (
        db.session.query(User)
        .outerjoin(
            Friendship,
            db.and_(
                u_alice.ID == Friendship.User_id,
                User.ID == Friendship.Friend_id,
            )
        )
        .filter(Friendship.ID == None)
        .filter(User.ID != u_alice.ID)
        .all()
    )
    for x in q:
        print(x)

Upvotes: 12

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