WIP 2

lib/shoulda/matchers/active_record/validate_uniqueness_of_matcher.rb

@@ -691,6 +691,8 @@ unsupported association type :#{reflection.macro}.
             previous_value.to_datetime.next
           elsif boolean_value?(previous_value)
             !previous_value
+          elsif previous_value.is_a?(ActiveRecord::Base)
+            previous_value.class.new
           else
             previous_value.to_s.next
           end

spec/support/unit/rails_application.rb

@@ -20,6 +20,7 @@ module UnitTests
       fs.within_project do
         install_gems
         remove_unwanted_gems
+        raise_errors_on_mass_assignment_violations
       end
     end
 
@@ -119,6 +120,14 @@ end
       end
     end
 
+    def raise_errors_on_mass_assignment_violations
+      fs.write 'config/initializers/protected_attributes.rb', <<-CONTENT
+        Rails.application.configure do
+          config.active_record.mass_assignment_sanitizer = :strict
+        end
+      CONTENT
+    end
+
     def run_command!(*args)
       Tests::CommandRunner.run!(*args)
     end