= cohort_scope

Provides cohorts (in the form of ActiveRecord scopes) that dynamically widen until they contain a certain number of records.

* <tt>big_cohort</tt> widens by finding the constraint that eliminates the most records and removing it.
* <tt>strict_cohort</tt> widens by eliminating constraints in order.

= Real-world use

This has been at use at http://carbon.brighterplanet.com since April 2010, where it helps sift through climate data to come up with meaningful emissions calculations.

= Quick start

Let's pretend the U.S. Census provided information about birthday and favorite color:

  class Citizen < ActiveRecord::Base
    extend CohortScope
    self.minimum_cohort_size = 1_000
  end

Now I need to run a calculation that ideally uses birthday and favorite color, but most importantly looks at a large cohort:

  Citizen.big_cohort :birthdate => (Date.parse('1980-01-01')..Date.parse('1990-01-01')), :favorite_color => 'heliotrope'
  # => [... a cohort of at least 1,000 records (otherwise it's empty),
            where everybody's favorite color MAY be heliotrope
            and everybody's birthday MAY be between 1980 and 1990
            (at least one of those constraints will hold) ...]

What if my calculation privileges favorite color? In other words, if you can't give me a cohort of minimum size within the birthday constraint, at least give me one where everybody loves heliotrope:

  ordered_constraints = ActiveSupport::OrderedHash.new
  ordered_constraints[:favorite_color] = 'heliotrope'
  ordered_constraints[:birthdate] = (Date.parse('1980-01-01')..Date.parse('1990-01-01'))

  Citizen.strict_cohort favorite_color_matters_most
  # => [... a cohort of at least 1,000 records (otherwise it's empty),
            where everybody's favorite color IS heliotrope
            and everybody's birthday MAY be between 1980 and 1990 ...]

= Wishlist

* support for ruby 1.9's implicitly ordered hashes
* support for constraining on <tt>IS NULL</tt> or <tt>IS NOT NULL</tt>

== Copyright

Copyright (c) 2010 Seamus Abshere and Andy Rossmeissl. See LICENSE for details.