allow AR caches to be cleared, clear them on class reloading

activerecord/lib/active_record/base.rb

@@ -750,6 +750,12 @@ module ActiveRecord #:nodoc:
         @arel_engine = @relation = nil
       end
 
+      def clear_cache! # :nodoc:
+        @@columns.clear
+        @@columns_hash.clear
+        @@arel_tables.clear
+      end
+
       def reset_column_cache # :nodoc:
         @@columns.delete table_name
         @@columns_hash.delete table_name

activerecord/lib/active_record/railtie.rb

@@ -72,6 +72,7 @@ module ActiveRecord
       ActiveSupport.on_load(:active_record) do
         ActionDispatch::Reloader.to_cleanup do
           ActiveRecord::Base.clear_reloadable_connections!
+          ActiveRecord::Base.clear_cache!
         end
       end
     end

activerecord/test/cases/base_test.rb

@@ -1553,6 +1553,14 @@ class BasicsTest < ActiveRecord::TestCase
     end
   end
 
+  def test_clear_cache!
+    # preheat cache
+    c1 = Post.columns
+    ActiveRecord::Base.clear_cache!
+    c2 = Post.columns
+    assert_not_equal c1, c2
+  end
+
   def test_default_scope_is_reset
     Object.const_set :UnloadablePost, Class.new(ActiveRecord::Base)
     UnloadablePost.table_name = 'posts'