Fix generator for non-association/columnar attrs. (#1290)

Rails allows for custom attributes, and these are often used with gems
such as `attr_encrypted`. Previously, the Dashboard generator would fail
with:

> undefined method `has_one?' for nil:NilClass (NoMethodError)

Instead of failing with an error, this ensures we rely on column names
instead of attributes names.

lib/generators/administrate/dashboard/dashboard_generator.rb

@@ -50,7 +50,9 @@ def namespace
       end
 
       def attributes
-        klass.reflections.keys + klass.attribute_names - redundant_attributes
+        klass.reflections.keys +
+          klass.columns.map(&:name) -
+          redundant_attributes
       end
 
       def form_attributes

spec/generators/dashboard_generator_spec.rb

@@ -349,6 +349,29 @@ class Ticket < ActiveRecord::Base
           remove_constants :Account, :Ticket
         end
       end
+
+      if ActiveRecord.version >= Gem::Version.new(5)
+        it "skips temporary attributes" do
+          begin
+            ActiveRecord::Schema.define do
+              create_table :accounts
+            end
+
+            class Account < ActiveRecord::Base
+              reset_column_information
+              attribute :tmp_attribute, :boolean
+            end
+
+            dashboard = file("app/dashboards/account_dashboard.rb")
+
+            run_generator ["account"]
+
+            expect(dashboard).not_to contain("tmp_attribute")
+          ensure
+            remove_constants :Account
+          end
+        end
+      end
     end
 
     describe "COLLECTION_ATTRIBUTES" do