Stick this in the lib directory and use it instead of SimpleDelegator to solve this issue: http://thepugautomatic.com/2014/03/simpledelegator-autoloading-issues-with-rails/

lib-rails_compatible_simple_delegator.rb

# See:
# * http://thepugautomatic.com/2014/03/simpledelegator-autoloading-issues-with-rails/
# * https://groups.google.com/forum/#!topic/rubyonrails-core/PjGUK72BmFA
# * https://gist.github.com/henrik/9314943

require "delegate"

class RailsCompatibleSimpleDelegator < SimpleDelegator
  def self.const_missing(name)
    if ::Object.const_defined?(name)
      # This is what SimpleDelegator usually does.
      # Load top-level constants even though SimpleDelegator inherits from BasicObject.
      ::Object.const_get(name)
    else
      # Rails autoloading.
      ::ActiveSupport::Dependencies.load_missing_constant(self, name)
    end
  end
end

unit-fixtures-my_fixture_delegator-subthing.rb

class MyFixtureDelegator::Subthing
end

unit-fixtures-my_fixture_delegator.rb

class MyFixtureDelegator < RailsCompatibleSimpleDelegator
end

unit-lib-rails_compatible_simple_delegator_spec.rb

require "spec_helper"
require "rails_compatible_simple_delegator"

# So Rails autoloading looks in unit/fixtures.
require "active_support/dependencies"
ActiveSupport::Dependencies.autoload_paths << File.join(File.dirname(__FILE__), "../fixtures")

describe RailsCompatibleSimpleDelegator do
  it "mostly works like SimpleDelegator" do
    delegator = RailsCompatibleSimpleDelegator.new("foo")
    expect(delegator.reverse).to eq "oof"
  end

  it "can look up standard library constants without an explicit '::' (like SimpleDelegator)" do
    expect(RailsCompatibleSimpleDelegator::File).to eq ::File
  end

  it "can autoload its own namespaced constants (like Rails autoloading)" do
    expect(MyFixtureDelegator::Subthing).to be_present
  end
end