Question

I was wondering if it’s possible to do something like follows:

Let’s say I have a Rails model, Foo, with a database attribute value. Foo belongs_to Bar, Bar has_many Foos.

In my model, I’d like to do something like:

class Foo < ActiveRecord::Base

  belongs_to :bar

  def self.average
    # return the value of all foos here
  end

end

Ideally I’d like to have this method return a value that matched the scope from which it was called, so that:

Foo.average # would return the average value of all foos

@bar = Bar.find(1)
@bar.foos.average # would return the average of all foos where bar_id == 1

Can such a thing be done, and if so, how? Thanks!

Answer 1

What you have will work as is, as long as you make sure to call methods on self instead of Foo in the body of the average method. When calling methods on a scope of Foo, self in the body of that method will be assigned to the scope object rather than Foo. Here’s a slightly more concrete example:

# app/models/club.rb
class Club < ActiveRecord::Base
  # name:string
  has_many :people
end

# app/models/person.rb
class Person < ActiveRecord::Base
  # club_id:integer, name:string, age:integer
  belongs_to :club

  def self.average_age
    # note that sum and count are being called on self, not Person
    sum('age') / count
  end
end

Let’s see what happens when we create some clubs and people:

$ rails console
Loading development environment (Rails 3.0.3)
irb(main):001:0> boys_club = Club.create(:name => 'boys')
irb(main):002:0> girls_club = Club.create(:name => 'girls')
irb(main):003:0> boys_club.people.create(:name => 'bob', :age => 20)
irb(main):004:0> boys_club.people.create(:name => 'joe', :age => 22)
irb(main):005:0> girls_club.people.create(:name => 'betty', :age => 30)
irb(main):006:0> Person.average_age
=> 24
irb(main):007:0> boys_club.people.average_age
=> 21
irb(main):008:0> Person.where("name LIKE 'b%'").average_age
=> 25