Use a scope by default on a Rails has_many relationship

In Rails 5, the following code works fine...

  class Order 
    scope :paid, -> { where status: %w[paid refunded] }
  end 

  class Store 
    has_many :paid_orders, -> { paid }, class_name: 'Order'
  end 

i just had a deep dive into ActiveRecord and it does not look like if this can be achieved with the current implementation of has_many. you can pass a block to :conditions but this is limited to returning a hash of conditions, not any kind of arel stuff.

a really simple and transparent way to achieve what you want (what i think you are trying to do) is to apply the scope at runtime:

  # foo.rb
  def bars
    super.baz
  end

this is far from what you are asking for, but it might just work ;)

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