rails模型的动态属性

Question

I'm wondering what is the best way to achieve what I want :p

I have a model 'ModelA' which has many 'ModelARate'. ModelARate contains a comment or a rate (or both).

I don't want to calculate the average rate each time a ModelA is shown. Because it's not cool for the database (accessing all ModelARate and getting rate ...) I need to know also the lowest and the highest rate.

How is the best way to achieve this in rails ? (without using a gem, I need to understand how it actually works)

I was thinking of adding a attribute rate_score in ModelA which is increasing when a ModelARate is created, and a method which return rate_score / modelarates.size For the lowest and highest rate, an attribute too ? There is no mechanism that can do this for me in rails ?

Thanks for any help

Answer 1

You can use the calculation method on Rails http://api.rubyonrails.org/classes/ActiveRecord/Calculations.html

For your models, given a ModelA record, you can

maximum = model_a.model_a_rates.maximum(:rate)
minimum = model_a.model_a_rates.minimum(:rate)
average = model_a.model_a_rates.average(:rate)

all of these calculations are done on the sql server so they are very fast so there's no need for you to worry about speed.