I'm trying to get my head around where to use the datastore for
business logic, and where to use python code. So just wanted to ask
two "which approach is better" questions:
Scenario 1. For example, let's say you have a list of items, and you
need to return both the selected item as well as the list of items.
For example - a person has a bunch of cars: [Honda, Ford, Porsche,
Yugo, Trabaunt, Dusenberg]. You have stored the Yugo key as the
person's 'active_car'.
You need to return: car, cars = Yugo, [Honda, Ford,
Porsche, Yugo, Trabaunt, Dusenberg]
Which approach is better to return the values...
Approach A: cars=person.car_set.fetch(1000)
car = db.get(stored_car_key)
return car, cars
Approach B: cars = person.car_set.fetch(1000)
car = [i for i in cars if i.key() ==
stored_car_key]
return car, cars
In other words - what's cheaper - the list comprehension, or the
db.get(). If the person has 1000 cars, does the answer change?
Scenario 2. I have a list of 300 people (In my case, there will never
be more than 1000) that I need to slice and dice in different ways.
a. Need them all, by last name, from California. b. Need people
between the ages of 25 and 35 in California. c. Need people over 300
lbs in California. Which approach should I use:
Approach A: Create multiple queries:
a. people =
state.people_set.order('last_name').fetch(1000)
b. people =
state.people_set.order('age').filter('age >', 35).etc.
c. people =
state.people_set.order(weight').filter('weight >', 300).etc.
Approach B: Memcache the entire list of people, and
list_comprehension them into submission. For example:
def return_people_by_last_name():
people = get_all_people_from_memcache()
sort_list = [(i.last_name, i) for i in people
if i.state == state]
sort_list.sort()
return [i[1] for i in sort_list]
def sort_people_by_weight():
similar to above...
In approach A, you'll be bearing the cost of additional indexes, as
well as bearing the cost that most of your returns will be hits to the
database. In approach B, you might be pulling 300 People from
memcache in order to return a single 300 pounder.
Answers to these two questions might give me a better sense of when to
hit the datastore for business logic, and when to process using python
code.
Thanks!
johnP
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