Sort by Count of Many to Many Relationship - SQLAlchemy

问题

I am using Flask-SQLAlchemy to to query my Postgres database.

I am currently trying to query for suggestions of titles with the following query:

res = Title.query.filter(Titles.name.ilike(searchstring)).limit(20)

So far so good.

Now I would like to order the results by the number of "subscribers" each Title object has.

I am aware of the following SO question: SQLAlchemy ordering by count on a many to many relationship however its solution did not work for me.

I am receiving the following error:

ProgrammingError: (ProgrammingError) column "publishers_1.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: ...itles_name, count(titles_users.user_id) AS total, publishers...

(Publisher has a one-to-many relationship with Title models)

I understand that the answer has something to do with subqueries

Below is a simplified example of my two models.

# Many-to-Many relationship for user and titles
titles_users = db.Table('titles_users',
    db.Column('user_id', db.Integer, db.ForeignKey('users.id')),
    db.Column('title_id', db.Integer, db.ForeignKey('titles.id'))
)

class User(db.Model, UserMixin):
    __tablename__ = 'users'
    # ids
    id = db.Column(db.Integer, primary_key=True)
    # Attributes
    email = db.Column(db.String(255), unique=True)
    full_name = db.Column(db.String(255))

    pull_list = db.relationship(
        'Title',
        secondary=titles_users,
        backref=db.backref('users', lazy='dynamic'),
        lazy='joined'
    )


class Title(db.Model):
    __tablename__ = 'titles'
    #: IDs
   id = db.Column(db.Integer(), primary_key=True)
   #: Attributes
   name = db.Column(db.String(255))

回答1:

Code below would work for the model you describe:

q = (db.session.query(Title, func.count(names_users.c.user_id).label("total"))
        .filter(Title.name.ilike(searchstring))
        .outerjoin(names_users).group_by(Title).order_by('total DESC')
    )
for x in q:
    print(x)

Your error, however, includes some other data like publisher or like. So if above is not helpful, you should add more information to your question.

来源：https://stackoverflow.com/questions/24565211/sort-by-count-of-many-to-many-relationship-sqlalchemy

标签

python

postgresql

Flask

sqlalchemy

flask-sqlalchemy