Use a scope by default on a Rails has_many relationship

In Rails 4, Associations have an optional scope parameter that accepts a lambda that is applied to the Relation (cf. the doc for ActiveRecord::Associations::ClassMethods)

class SolarSystem < ActiveRecord::Base
  has_many :planets, -> { life_supporting }
end

class Planet < ActiveRecord::Base
  scope :life_supporting, -> { where('distance_from_sun > ?', 5).order('diameter ASC') }
end

In Rails 3, the where_values workaround can sometimes be improved by using where_values_hash that handles better scopes where conditions are defined by multiple where or by a hash (not the case here).

has_many :planets, conditions: Planet.life_supporting.where_values_hash