Fix enum fields losing class-level enumerations in the filters dropdown

enum_method asked bindings[:object].class whether it responds to
"#{name}_enum". With no object bound that reads NilClass.respond_to?, which is
always false, so a model defining the enumeration as a class method fell back to
the attribute name. #enum then read the attribute off a throwaway record and
returned nil, and filter_operators raised NoMethodError on nil.

The filters dropdown renders each field with a view but no object
(field.with(view: self) in index.html.erb), so every enum field on such a model
crashed the index page.

Fall back to the model itself when nothing is bound. Existing specs missed this
because they all bind an object before asking.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: mshibuya

lib/rails_admin/config/fields/types/enum.rb

@@ -21,7 +21,16 @@ class Enum < RailsAdmin::Config::Fields::Base
           end
 
           register_instance_option :enum_method do
-            @enum_method ||= bindings[:object].class.respond_to?("#{name}_enum") || (bindings[:object] || abstract_model.dummy_record).respond_to?("#{name}_enum") ? "#{name}_enum" : name
+            # Fall back to the model when no object is bound, which is how the
+            # filters dropdown renders fields. Asking nil for its class used to
+            # look the method up on NilClass, hiding class-level enumerations.
+            @enum_method ||=
+              if (bindings[:object]&.class || abstract_model.model).respond_to?("#{name}_enum") ||
+                 (bindings[:object] || abstract_model.dummy_record).respond_to?("#{name}_enum")
+                "#{name}_enum"
+              else
+                name
+              end
           end
 
           register_instance_option :enum do

spec/rails_admin/config/fields/types/enum_spec.rb

@@ -45,6 +45,37 @@ def color_enum
     end
   end
 
+  describe "when only the class responds to '\#{method}_enum'" do
+    before do
+      Team.singleton_class.class_eval do
+        def manager_enum
+          %w[Alice Bob]
+        end
+      end
+      RailsAdmin.config Team do
+        list do
+          field :manager, :enum
+        end
+      end
+    end
+
+    after do
+      Team.singleton_class.send(:remove_method, :manager_enum)
+    end
+
+    let(:field) { RailsAdmin.config(Team).list.fields.detect { |f| f.name == :manager } }
+
+    # The filters dropdown renders fields bound to a view but to no object,
+    # so the enumeration has to resolve without one.
+    it 'resolves the enumeration without a bound object' do
+      expect(field.with(view: nil).enum).to eq %w[Alice Bob]
+    end
+
+    it 'builds filter options without a bound object' do
+      expect { field.with(view: nil).filter_options }.not_to raise_error
+    end
+  end
+
   describe 'the enum instance method' do
     before do
       Team.class_eval do