Extract test to new file: ar_id_map_test.rb

This test is quite strange, and has very little explanation.

In an effort to understand it better, move it and its context to a new file.

In particular, reopening the ActiveRecord::IdentityMap module inside a test
is unusual. More importantly, it's not clear why we're testing a method that
we do not implement.

.rubocop_todo.yml

@@ -59,7 +59,7 @@ Style/ClassAndModuleChildren:
   Exclude:
     - 'test/functional/sinatra_test.rb'
     - 'test/test_helper.rb'
-    - 'test/unit/model_test.rb'
+    - 'test/unit/ar_id_map_test.rb'
 
 # Offense count: 1
 # Cop supports --auto-correct.

test/unit/ar_id_map_test.rb

@@ -0,0 +1,26 @@
+require 'test_helper'
+
+class ARIdMapTest < ActiveSupport::TestCase
+  setup do
+    @widget = Widget.new
+    @widget.update_attributes :name => 'Henry', :created_at => Time.now - 1.day
+    @widget.update_attributes :name => 'Harry'
+  end
+
+  if defined?(ActiveRecord::IdentityMap) && ActiveRecord::IdentityMap.respond_to?(:without)
+    should 'not clobber the IdentityMap when reifying' do
+      module ActiveRecord::IdentityMap
+        class << self
+          alias __without without
+          def without(&block)
+            @unclobbered = true
+            __without(&block)
+          end
+        end
+      end
+
+      @widget.versions.last.reify
+      assert ActiveRecord::IdentityMap.instance_variable_get("@unclobbered")
+    end
+  end
+end

test/unit/model_test.rb

@@ -342,23 +342,6 @@ class HasPaperTrailModelTest < ActiveSupport::TestCase
           assert_equal ['Henry', 'Harry'], @widget.versions.last.changeset['name']
         end
 
-        if defined?(ActiveRecord::IdentityMap) && ActiveRecord::IdentityMap.respond_to?(:without)
-          should 'not clobber the IdentityMap when reifying' do
-            module ActiveRecord::IdentityMap
-              class << self
-                alias :__without :without
-                def without(&block)
-                  @unclobbered = true
-                  __without(&block)
-                end
-              end
-            end
-
-            @widget.versions.last.reify
-            assert ActiveRecord::IdentityMap.instance_variable_get("@unclobbered")
-          end
-        end
-
         context 'and has one associated object' do
           setup do
             @wotsit = @widget.create_wotsit :name => 'John'
@@ -381,7 +364,6 @@ class HasPaperTrailModelTest < ActiveSupport::TestCase
           end
         end
 
-
         context 'and has many associated objects' do
           setup do
             @f0 = @widget.fluxors.create :name => 'f-zero'