Question

I have a Project model and a Developer model. I have the concept of calculating the “interestingness” for a project for a particular developer:

class Project < ActiveRecord::Base
  def interestingness_for(developer)
    some_integer_based_on_some_calculations
  end
end

I think it would be neat, instead of having something like Project.order_by_interestingness_for(bill), to be able to say

Project.order(:interestingness, :developer => bill)

and have it be a scope as opposed to just a function, so I can do stuff like

Project.order(:interestingness, :developer => bill).limit(10)

I don’t know how to do this, though, because it’s not obvious to me how to override a scope. Any advice?

Answer 1

Assuming you will not need to use the standard ActiveRecord order query method for the Project class, you can override it like any other class method:

def self.order(type, options)
  self.send(:"special_#{type}_calculation_via_scopes", options)
end

Then the trick is to ensure you create the needed calculation methods (which will vary according to your interestingness and other algorithms). And that the calculation methods only use scopes or other AR query interface methods. If you aren’t comfortable converting the method logic to a SQL equivalent using the query interface, you can try using the Squeel DSL gem which can potentially work with the method directly depending on your specific calculation.

If you may be needing the classic order method (and this is usually a safe assumption), then don’t override it. Either create a proxy non-ActiveRecord object for this purpose, or use a different naming convention.

If you really want to, you can use aliasing to achieve a similar effect, but it may have unintended consequences for the long term if the second argument (‘options’ in this case) suddenly takes on another meaning as Rails progresses. Here is an example of what you can use:

def self.order_with_options(type, options = nil)
  if options.nil?
    order_without_options(type)
  else
    self.send(:"special_#{type}_calculation_via_scopes", options)
  end
end    
class << self
  alias_method_chain :order, :options
end