Question

I have the following models in sqlalchemy:

class Magazine(db.Model):

    id= db.Column(db.Integer, primary_key=True)
    name= db.Column(db.String(50))
    issues= db.relationship('Issue', backref='magazine', lazy='dynamic')

class Issue(db.Model):
    id= db.Column(db.Integer, primary_key=True)
    magazine_id= db.Column(db.Integer, db.ForeignKey('magazine.id'))
    date= db.Column(db.Date)

Magazines holds names of magazines and issues holds the issues of each magazine. These are weekly magazines, so the date field in Issue is the day of the issue.

I am try to build a query to select all magazines who do not have an issue for a specific date. How’s the easiest way to do it?

Answer 1

Since this is tagged sql, I presume you’re OK with proper SQL, not whatever SQLAlchemy uses. This should be a hint about how to express the same thing in SQLAlchemy anyway.

SELECT *
FROM magazine m
WHERE NOT EXISTS (
  SELECT 1 FROM issue i WHERE i.magazine_id = m.id AND i."date" = $1
);

$1 is the date parameter; in Python it might be %s or ? … whatever your language’s parameter placeholder is.

BTW, "date" is a terrible name for a column. It’ll have to be double-quoted everywhere because it’s a typename and an SQL reserved word.