100% test coverage 🎉

app/models/element/null.rb

@@ -2,4 +2,24 @@ class Element::Null
   def displayed?
     false
   end
+
+  def self.has_query_constraints?
+    false
+  end
+
+  def self.composite_primary_key?
+    nil
+  end
+
+  def self.primary_key
+    "id"
+  end
+
+  def _read_attribute(attr)
+    nil
+  end
+
+  def self.polymorphic_name
+    "Element::Null"
+  end
 end

test/models/element/radio_button_test.rb

@@ -0,0 +1,9 @@
+class Element::RadioButtonTest < ActiveSupport::TestCase
+  setup do
+    @radio_button = Element::RadioButton.new
+  end
+
+  test "radio button is displayed" do
+    assert @radio_button.displayed?
+  end
+end

test/models/element_test.rb

@@ -27,4 +27,18 @@
 require "test_helper"
 
 class ElementTest < ActiveSupport::TestCase
+  test "#null returns a new Element with Null variant" do
+    element = Element.null
+    assert_equal "Element::Null", element.variant_type
+  end
+
+  test "#variant returns Null variant when super is nil" do
+    element = Element.new
+    assert_instance_of Element::Null, element.variant
+  end
+
+  test "#displayed? delegates to variant" do
+    element = Element.new(variant: Element::RadioButton.new)
+    assert element.displayed?
+  end
 end