allow_value: Fix compatibility with enum columns

When used against an attribute that's an enum, `allow_value` will raise
an AttributeValueChangedError. This commit prevents that from happening.

NEWS.md

@@ -1,5 +1,17 @@
 # HEAD
 
+### Bug fixes
+
+* Update `validate_numericality_of` so that submatchers are applied lazily
+  instead of immediately. Previously, qualifiers were order-dependent, meaning
+  that if you used `strict` before you used, say, `odd`, then `strict` wouldn't
+  actually apply to `odd`. Now the order that you specify qualifiers doesn't
+  matter.
+
+* Fix `allow_value` so that it does not raise an AttributeChangedValueError
+  (formerly CouldNotSetAttributeError) when used against an attribute that is an
+  enum in an ActiveRecord model.
+
 ### Improvements
 
 * Improve failure messages and descriptions of all matchers across the board so
@@ -7,15 +19,12 @@
   (You'll see a huge difference in the output of the numericality and uniqueness
   matchers in particular.)
 
+* The CouldNotSetAttributeError that you have probably seen is now called
+  AttributeChangedValueError.
+
 * Matchers now raise an error if any attributes that the matcher is attempting
   to set do not exist on the model.
 
-* Update `validate_numericality_of` so that submatchers are applied lazily
-  instead of immediately. Previously, qualifiers were order-dependent, meaning
-  that if you used `strict` before you used, say, `odd`, then `strict` wouldn't
-  actually apply to `odd`. Now the order that you specify qualifiers doesn't
-  matter.
-
 * Update `validate_numericality_of` so that it doesn't always run all of the
   submatchers, but stops on the first one that fails. Since failure messages
   now contain information as to what value the matcher set on the attribute when

lib/shoulda/matchers/active_model/allow_value_matcher/attribute_setter.rb

@@ -78,7 +78,7 @@ module Shoulda
 
             @result_of_checking = successful_check
 
-            if attribute_changed_value? && !ignoring_interference_by_writer?
+            if raise_attribute_changed_value_error?
               @result_of_setting = attribute_changed_value_error
               false
             else
@@ -156,6 +156,33 @@ module Shoulda
             !!@ignoring_interference_by_writer
           end
 
+          def raise_attribute_changed_value_error?
+            attribute_changed_value? &&
+              !(attribute_is_an_enum? && value_read_is_expected_for_an_enum?) &&
+              !ignoring_interference_by_writer?
+          end
+
+          def attribute_is_an_enum?
+            enum_values.any?
+          end
+
+          def value_read_is_expected_for_an_enum?
+            enum_values.key?(value_read) &&
+              enum_values[value_read] == value_written
+          end
+
+          def enum_values
+            defined_enums.fetch(attribute_name.to_s, {})
+          end
+
+          def defined_enums
+            if model.respond_to?(:defined_enums)
+              model.defined_enums
+            else
+              {}
+            end
+          end
+
           def successful_check
             SuccessfulCheck.new
           end
@@ -192,6 +219,10 @@ module Shoulda
           def active_resource_object?
             object.respond_to?(:known_attributes)
           end
+
+          def model
+            object.class
+          end
         end
       end
     end

spec/unit/shoulda/matchers/active_model/allow_value_matcher_spec.rb

@@ -641,4 +641,18 @@ value", but that attribute does not exist.
       end
     end
   end
+
+  if active_record_supports_enum?
+    context 'given an ActiveRecord model' do
+      context 'where the attribute under test is an enum and the given value is a value in that enum' do
+        it 'accepts' do
+          model = define_model('Shipment', status: :integer) do
+            enum status: { pending: 1, shipped: 2, delivered: 3 }
+          end
+
+          expect(model.new).to allow_value(1).for(:status)
+        end
+      end
+    end
+  end
 end