Fix enum attribute name verification

Fix `define_enum_for` so that it actually tests that the attribute is
present in the list of defined enums, as you could fool it by merely
defining a class method that was the pluralized version of the attribute
name. In the same vein, passing a pluralized version of the attribute
name to `define_enum_for` would erroneously pass, and now it fails.

NEWS.md

@@ -31,6 +31,14 @@
     attributes is a UUID column that ends in an "f", the matcher is able to
     generate a proper "next" value without erroring.
 
+* Fix `define_enum_for` so that it actually tests that the attribute is present
+  in the list of defined enums, as you could fool it by merely defining a class
+  method that was the pluralized version of the attribute name. In the same
+  vein, passing a pluralized version of the attribute name to `define_enum_for`
+  would erroneously pass, and now it fails. ([#641])
+
+[#641]: https://github.com/thoughtbot/shoulda-matchers/pull/641
+
 # 2.8.0
 
 ### Deprecations

lib/shoulda/matchers/active_record/define_enum_for_matcher.rb

@@ -99,16 +99,12 @@ def expected_enum_values
           hashify(options[:expected_enum_values]).with_indifferent_access
         end
 
-        def enum_method
-          attribute_name.to_s.pluralize
-        end
-
         def actual_enum_values
-          model.class.send(enum_method)
+          model.class.send(attribute_name.to_s.pluralize)
         end
 
         def enum_defined?
-          model.class.respond_to?(enum_method)
+          model.defined_enums.include?(attribute_name.to_s)
         end
 
         def enum_values_match?

spec/unit/shoulda/matchers/active_record/define_enum_for_matcher_spec.rb

@@ -2,6 +2,31 @@
 
 describe Shoulda::Matchers::ActiveRecord::DefineEnumForMatcher, type: :model do
   if active_record_supports_enum?
+    context 'if the attribute is given in plural form accidentally' do
+      it 'rejects' do
+        record = record_with_array_values
+        plural_enum_attribute = enum_attribute.to_s.pluralize
+        message = "Expected #{record.class} to define :#{plural_enum_attribute} as an enum"
+        assertion = -> {
+          expect(record).to define_enum_for(plural_enum_attribute)
+        }
+        expect(&assertion).to fail_with_message(message)
+      end
+    end
+
+    context 'if a method to hold enum values exists on the model but was not created via the enum macro' do
+      it 'rejects' do
+        model = define_model :example do
+          def self.statuses; end
+        end
+        message = "Expected #{model} to define :statuses as an enum"
+        assertion = -> {
+          expect(model.new).to define_enum_for(:statuses)
+        }
+        expect(&assertion).to fail_with_message(message)
+      end
+    end
+
     describe "with only the attribute name specified" do
       it "accepts a record where the attribute is defined as an enum" do
         expect(record_with_array_values).to define_enum_for(enum_attribute)