Using joins and conditions in ActiveRecord scopes. (This was hard for me to find documentation on.)

activerecord_scope_conditions.rb

class Person < ActiveRecord::Base

  # This manual SQL query...
  scope :allowed_to_eat_cheese, joins(
    <<-SQL
    INNER JOIN cities ON people.city_id  = cities.id
    INNER JOIN states ON cities.state_id = states.id
    SQL
    ).where('states.allows_cheese_consumption' = 'Yeppers')

  # ... can be automated by this ActiveRecord syntax
  scope :allowed_to_eat_cheese, 
    joins: {:city => [:state]}, conditions: {'states.allows_cheese_consumption' => 'Yeppers' }

end