database field created by a relational field in flask sqlalchemy models

Question

say...
Nation has states has cities has schools has students.

if my database is set-up like that, and I have a field in schools that is a boolean goodOrBadStudent, is there an easy way to calculate how many good students there are in each table?

NationGoodStudents=y
StateGoodStudents=x
and so on... ?

this is my real data but the idea is the same

class Projects(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    total = db.Column(db.Integer)
    percentDeadline = db.Column(db.Integer)
    percent = db.Column(db.Integer)
    name = db.Column(db.String(50))
    goals = db.relationship('Goals', backref='proj',
                                lazy='dynamic')
    def __repr__(self):
        return '<Project %r>' % (self.name)

class Goals(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    goal = db.Column(db.String(50))
    project_id = db.Column(db.Integer, db.ForeignKey('projects.id'))
    strategies = db.relationship('Strategies', backref='goa',
                                lazy='dynamic')

    def __repr__(self):
        return '<Goals %r>' % (self.goal)
class Strategies(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    strategy = db.Column(db.String(50))
    goal_id = db.Column(db.Integer, db.ForeignKey('goals.id'))
    tasks = db.relationship('Tasks', backref='strat',
                                lazy='dynamic')
    def __repr__(self):
        return '<Strategy %r>' % (self.strategy)

class Tasks(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    task = db.Column(db.String(50))
    note = db.Column(db.String(400))  
    complete = db.Column(db.Boolean())
    staff = db.Column(db.String(30))
    deadline = db.Column(db.Date)
    completeDate = db.Column(db.Date)
    created = db.Column(db.Date)
    strategy_id = db.Column(db.Integer, db.ForeignKey('strategies.id'))

    def __repr__(self):
        return '<Tasks %r>' % (self.task)

so where would i do the calculation? I hope not in the view, because that is cumbersome.

here is what i started doing to get the percents, but probably better to pass x y and total

P=models.Projects.query.all()
for p in P:  
    y=0     
    x=0
    total=0
    project=models.Projects.query.filter_by(name=p.name).first() 
    G=project.goals.all()
    for g in G:
        pgoal=models.Goals.query.filter_by(goal=g.goal).first() 
        S=pgoal.strategies.all()
        for s in S:
            pstrat=models.Strategies.query.filter_by(strategy=s.strategy).first() 
            T=pstrat.tasks.all()
            for t in T:
                if t.completeDate != None and t.deadline != None:
                    deadlineDiff= (t.deadline - t.completeDate) > datetime.timedelta(days=0)
                    print deadlineDiff, ' this is the deadlineDiff variable for ',t.task
                    if deadlineDiff==True:
                        y+=1
                total+=1
                if t.complete==True:
                    x+=1
    y=float(y*1.00)
    x=float(x*1.00)
    total=float(total*1.00)
    if x == 0 or total ==0:
        p.percent=0
        db.session.commit()
    else:
        p.percent=float(x/total*100.0)
        db.session.commit()   
    if y == 0 or x ==0:
        p.percentDeadline=0
        db.session.commit()
    else:
        p.percentDeadline=float(y/x*100.0)
        db.session.commit()     

I guess i would commit x, y and total at each level?

Answer 1

In my opinion, such calculation is best performed directly on the SQL side. Using sqlalchemy query below shall return a list of tuples (project_id, x, y, total) as these are defined in your sample code. Given you have all proper indices on your database, this query should be very fast, and you could use this to calculate the progress of each given project on the fly without even storing the results in the database.

q_sum = (session.query(
            Projects.id.label("project_id"),
            func.sum(case([(Tasks.complete == True, 1)], else_=0)).label("x"),
            func.sum(case([(and_(
                Tasks.deadline != None,
                Tasks.completeDate != None,
                Tasks.deadline > Tasks.completeDate), 1)],
                else_=0)).label("y"),
            func.count(Tasks.id).label("total"),
            )
        .join(Goals, Projects.goals)
        .join(Strategies, Goals.strategies)
        .join(Tasks, Strategies.tasks)
        .group_by(Projects.id)
        )

# (project_id, x, y, total)
for p in q_sum:
    print(p)

If you need to get this only for specific project, just add .filter(Project.id == my_project_id) to the query.