Initializer file to help patch Rails 3.2.* has_many associations when using Ruby 2.2. See https://github.com/rails/rails/issues/18991

activerecord_patch.rb

# For Rails 3.2.*
#
if Rails.version =~ /^3.2/
  # The call to has_cached_counter? throws an exception on the 3.2.* version of rails.
  # See https://github.com/rails/rails/issues/18991
  ActiveRecord::Associations::HasManyThroughAssociation.class_eval do
    def size
      if has_cached_counter?(reflection)
        owner.send(:read_attribute, cached_counter_attribute_name)
      elsif loaded?
        target.size
      else
        count
      end
    end

    def insert_record(record, validate = true, raise = false)
      ensure_not_nested

      if record.new_record?
        if raise
          record.save!(:validate => validate)
        else
          return unless record.save(:validate => validate)
        end
      end

      save_through_record(record)
      update_counter(1, reflection)
      record
    end
  end

  ActiveRecord::Associations::HasManyAssociation.class_eval do

  private

    def count_records
      count = if has_cached_counter?(reflection)
        owner.send(:read_attribute, cached_counter_attribute_name)
      elsif options[:counter_sql] || options[:finder_sql]
        reflection.klass.count_by_sql(custom_counter_sql)
      else
        scoped.count
      end

      # If there's nothing in the database and @target has no new records
      # we are certain the current target is an empty array. This is a
      # documented side-effect of the method that may avoid an extra SELECT.
      @target ||= [] and loaded! if count == 0

      [options[:limit], count].compact.min
    end
  end
end

Another option is to upgrade to the latest stable branch version for 3.2:

gem 'rails', github: 'rails/rails', branch: '3-2-stable'