Override Admin::ResourceController#parent method

This is not very nice but it's needed to avoid the use of `find_by_slug`, which
doesn't work with Globalize 5.1 because it removes the slug column.

This is only needed on Solidus versions from 2.0 to 2.2 because it's already
fixed in 2.3.

app/controllers/spree/admin/resource_controller_decorator.rb

@@ -0,0 +1,13 @@
+module SolidusGlobalize::ResourceControllerDecorator
+  def parent
+    if parent_data.present?
+      @parent ||= parent_data[:model_class].find_by(parent_data[:find_by] => params["#{model_name}_id"])
+      instance_variable_set("@#{model_name}", @parent)
+    end
+  end
+end
+
+if SolidusSupport.solidus_gem_version >= Gem::Version.new("2.0") &&
+   SolidusSupport.solidus_gem_version < Gem::Version.new("2.3")
+  Spree::Admin::ResourceController.prepend(SolidusGlobalize::ResourceControllerDecorator)
+end