feat: When an unrelated error is seen with negated allow_value, give a hint

lib/shoulda/matchers/active_model/allow_value_matcher.rb

@@ -402,6 +402,10 @@ def does_not_match?(instance)
           @result.nil?
         end
 
+        def has_any_errors?
+          validator.record.errors.any?
+        end
+
         def failure_message
           attribute_setter = result.attribute_setter
 
@@ -480,6 +484,11 @@ def failure_message_when_negated # rubocop:disable Metrics/MethodLength
                   message << " it produced these validation errors instead:\n\n"
                   message << validator.all_formatted_validation_error_messages
                 end
+              elsif validator.has_any_errors?
+                message << ", placing a validation error on :#{attribute_setter.attribute_name}"
+                message << '. The Example was invalid,'
+                message << " but it had errors involving other attributes:\n\n"
+                message << validator.all_formatted_validation_error_messages
               else
                 message << ', but it was valid instead.'
               end

lib/shoulda/matchers/active_model/validator.rb

@@ -25,6 +25,10 @@ def has_messages?
           messages.any?
         end
 
+        def has_any_errors?
+          record.errors.any?
+        end
+
         def captured_validation_exception?
           @captured_validation_exception
         end

spec/support/unit/helpers/allow_value_matcher_helpers.rb

@@ -7,6 +7,10 @@ def builder_for_record_with_different_error_attribute(options = {})
       RecordWithDifferentErrorAttributeBuilder.new(options)
     end
 
+    def builder_for_record_with_unrelated_error(options = {})
+      RecordWithUnrelatedErrorBuilder.new(options)
+    end
+
     def builder_for_record_with_different_error_attribute_using_i18n(options = {})
       builder = builder_for_record_with_different_error_attribute(options)
       RecordBuilderWithI18nValidationMessage.new(builder)

spec/support/unit/record_with_unrelated_error_builder.rb

@@ -0,0 +1,90 @@
+require_relative 'helpers/model_builder'
+
+module UnitTests
+  class RecordWithUnrelatedErrorBuilder
+    include ModelBuilder
+
+    def initialize(options)
+      @options = options.reverse_merge(default_options)
+    end
+
+    def attribute_that_receives_error
+      options[:attribute_that_receives_error]
+    end
+
+    def attribute_to_validate
+      options[:attribute_to_validate]
+    end
+
+    def message
+      options[:message]
+    end
+
+    def message=(message)
+      options[:message] = message
+    end
+
+    def model
+      @_model ||= create_model
+    end
+
+    def model_name
+      'Example'
+    end
+
+    def record
+      model.new
+    end
+
+    def valid_value
+      'some value'
+    end
+
+    protected
+
+    attr_reader :options
+
+    private
+
+    def context
+      {
+        validation_method_name: validation_method_name,
+        valid_value: valid_value,
+        attribute_to_validate: attribute_to_validate,
+        attribute_that_receives_error: attribute_that_receives_error,
+        message: message,
+      }
+    end
+
+    def create_model
+      _context = context
+
+      define_model model_name, model_columns do
+        validate _context[:validation_method_name]
+
+        define_method(_context[:validation_method_name]) do
+          errors.add(_context[:attribute_that_receives_error], _context[:message])
+        end
+      end
+    end
+
+    def validation_method_name
+      :custom_validation
+    end
+
+    def model_columns
+      {
+        attribute_to_validate => :string,
+        attribute_that_receives_error => :string,
+      }
+    end
+
+    def default_options
+      {
+        attribute_that_receives_error: :attribute_that_receives_error,
+        attribute_to_validate: :attribute_to_validate,
+        message: 'some message',
+      }
+    end
+  end
+end

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

@@ -353,6 +353,28 @@
 
   context 'when the attribute being validated is different than the attribute that receives the validation error' do
     include UnitTests::AllowValueMatcherHelpers
+    context 'when no validation message was provided directly' do
+      context 'when asserting the negative' do
+        it 'rejects with an appropriate failure message' do
+          builder = builder_for_record_with_unrelated_error
+          assertion = lambda do
+            expect(builder.record).
+              not_to allow_value(builder.valid_value).
+              for(builder.attribute_to_validate)
+          end
+
+          message = <<-MESSAGE
+After setting :attribute_to_validate to ‹"some value"›, the matcher
+expected the Example to be invalid, placing a validation error on
+:attribute_to_validate. The Example was invalid, but it had errors
+involving other attributes:
+
+* attribute_that_receives_error: ["some message"]
+          MESSAGE
+          expect(&assertion).to fail_with_message(message)
+        end
+      end
+    end
 
     context 'when the validation error message was provided directly' do
       context 'given a valid value' do

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

@@ -890,7 +890,7 @@ def validation_matcher_scenario_args
     context 'against a polymorphic association' do
       it 'matches when the subject configures validate_inclusion_of' do
         define_model(:issue, severity_id: :integer, severity_type: :string) do
-          belongs_to :severity, polymorphic: true
+          belongs_to :severity, polymorphic: true, optional: true
           validates_inclusion_of :severity_type, in: %w(Low Medium High)
         end
         define_model(:high)
@@ -902,7 +902,7 @@ def validation_matcher_scenario_args
 
       it 'does not match when subject does not set validate_inclusion_of' do
         define_model(:issue, severity_id: :integer, severity_type: :string) do
-          belongs_to :severity, polymorphic: true
+          belongs_to :severity, polymorphic: true, optional: true
         end
         define_model(:high)
         define_model(:medium)