concerning learns how to prepend the concern

jasonkarns · kaspth · commit 8543974afa8c · 2020-02-10T02:23:33.000+01:00
In rare instances, a module needs to be prepended into the ancestor
hierarchy, not simply included. In cases where this is necessary, and it
is desirable to use an inline concern, it would be beneficial to be able
to instruct `concerning` that the concern should be prepended.

This is now possible, by providing `prepend: true` kwarg to
`concerning`. (It is false by default.)
diff --git a/activesupport/lib/active_support/core_ext/module/concerning.rb b/activesupport/lib/active_support/core_ext/module/concerning.rb
@@ -106,8 +106,9 @@ class Module
   # * stop leaning on protected/private for crude "this is internal stuff" modularity.
   module Concerning
     # Define a new concern and mix it in.
-    def concerning(topic, &block)
-      include concern(topic, &block)
+    def concerning(topic, prepend: false, &block)
+      method = prepend ? :prepend : :include
+      __send__(method, concern(topic, &block))
     end
 
     # A low-cruft shortcut to define a concern.
diff --git a/activesupport/test/core_ext/module/concerning_test.rb b/activesupport/test/core_ext/module/concerning_test.rb
@@ -8,6 +8,18 @@ def test_concerning_declares_a_concern_and_includes_it_immediately
     klass = Class.new { concerning(:Foo) { } }
     assert_includes klass.ancestors, klass::Foo, klass.ancestors.inspect
   end
+
+  def test_concerning_can_prepend_concern
+    klass = Class.new do
+      def hi; "self"; end
+
+      concerning(:Foo, prepend: true) do
+        def hi; "hello, #{super}"; end
+      end
+    end
+
+    assert_equal "hello, self", klass.new.hi
+  end
 end
 
 class ModuleConcernTest < ActiveSupport::TestCase
