Eager load an association that uses interpolation for a key constraint

app__controllers__tabs_controller.rb

class TabsController < ApplicationController
  def index
    @tabs = Tab.all :include => :available_balance
  end
end

app__models__balance.rb

class Balance < ActiveRecord::Base
  belongs_to :customer, :class_name => 'User'
  belongs_to :merchant, :class_name => 'User'
end

app__models__tab.rb

class Tab < ActiveRecord::Base
  belongs_to :customer, :class_name => 'User'
  belongs_to :merchant, :class_name => 'User'

  has_one :available_balance, :class_name => 'Balance', :primary_key => :customer_id, :foreign_key => :customer_id, :conditions => 'merchant_id in (#{available_balance_merchant_ids})'
  def self.available_balance_merchant_ids; @records_to_preload.map(&:merchant_id).join ',' end # called when eager loading available_balance
  def available_balance_merchant_ids; merchant_id end # called when lazy loading available_balance
end

config__initializers__eager_load_association_with_interpolation.rb

# This patch enables access to the records being preloaded when eager loading
# an association whose conditions use interpolation to call class methods.

module ActiveRecord::AssociationPreload::ClassMethods
  [:has_and_belongs_to_many, :has_one, :has_many, :belongs_to].each do |type|
    define_method "preload_#{type}_association_with_access_to_records" do |records, reflection, *args|
      @records_to_preload_for_reflection ||= {}
      @records_to_preload_for_reflection[reflection] = records
      send "preload_#{type}_association_without_access_to_records", records, reflection, *args
      @records_to_preload_for_reflection.delete reflection
    end
    alias_method_chain "preload_#{type}_association", :access_to_records
  end

  def append_conditions_with_access_to_records reflection, *args
    @records_to_preload = @records_to_preload_for_reflection[reflection]
    sql = append_conditions_without_access_to_records reflection, *args
    remove_instance_variable :@records_to_preload
    sql
  end
  alias_method_chain :append_conditions, :access_to_records
end