CODEWITHSUNDEEP
pythonormponyorm
tonycpsu
tonycpsu

Reputation: 427

Are calculated/derived fields fully supported in Pony ORM?

Suppose I have the following schema in Pony ORM:

from pony.orm import *

db = Database("postgres", database='foo')

class Job(db.Entity):

    job_id = PrimaryKey(int, auto=True)
    job_name = Required(str)
    base_salary = Required(int)
    multiplier = Required(int, default=1000)
    people = Set(lambda: Person)

class Person(db.Entity):

    person_id = PrimaryKey(int, auto=True)
    name = Required(str)
    job = Required(lambda: Job)
    experience = Required(int)

I would like the Person entity to have a salary attribute that's equal to:

Job.base_salary + (Person.experience * Job.multiplier)

My first thought was to add a property to the Person entity like so:

@property
def salary(self):
    return self.job.base_salary + (self.experience * self.job.multiplier)

This works for a simple query:

j1 = Job(job_name = "Astronaut", base_salary = 80000, multiplier = 5000)
j2 = Job(job_name = "Butcher", base_salary = 40000, multiplier = 2000)
j3 = Job(job_name = "Chef", base_salary = 30000)

for i, name in enumerate(["Alice", "Bob", "Carol"]):
    p = Person(name = name, job=j1, experience = i)

for i, name in enumerate(["Dave", "Erin"]):
    p = Person(name = name, job=j2, experience = i)

for i, name in enumerate(["Frank", "Gwen"]):
    p = Person(name = name, job=j3, experience = i)

for p in select(p for p in Person):
    print p.name, p.experience, p.salary

Prints:

Alice 2 90000
Bob 4 100000
Carol 6 110000
Dave 2 44000
Erin 4 48000
Frank 2 32000
Gwen 4 34000

But if I try something like this:

for j in select((j.job_name, avg(j.people.salary)) for j in Job):
    print j

Perhaps unsurprisingly, I get:

AttributeError: j.people.salary

since salary isn't a "real" attribute. Is there a way to do this kind of thing that would allow calculated fields to be treated as first-class entities that can have normal aggregation / calculations done on them?

Upvotes: 4

Views: 425

Answers (1)

Alexander Kozlovsky
Alexander Kozlovsky

Reputation: 4849

Thanks for the suggestion!

Currently (Feb 2015) Pony ORM does not support calculated fields, but this feature can be implemented relatively easily, because Pony already have the ability to translate lambdas to SQL. I hope we can add calculated fields soon.

Upvotes: 3

Related Questions