Merge pull request rails#52983 from fatkodima/validate-uniqueness-with-singleton-class

Fix uniqueness validations defined on singleton classes

Author: rafaelfranca

activerecord/lib/active_record/validations/uniqueness.rb

@@ -14,6 +14,7 @@ def initialize(options)
         end
         super
         @klass = options[:class]
+        @klass = @klass.superclass if @klass.singleton_class?
       end
 
       def validate_each(record, attribute, value)

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -106,6 +106,16 @@ def test_validate_uniqueness
     assert t2.save, "Should now save t2 as unique"
   end
 
+  def test_validate_uniqueness_with_singleton_class
+    Topic.create!(title: "abc")
+    t2 = Topic.new(title: "abc")
+    t2.singleton_class.validates(:title, uniqueness: true)
+    assert_not_predicate t2, :valid?
+
+    t3 = Topic.new(title: "abc")
+    assert_predicate t3, :valid?
+  end
+
   def test_validate_uniqueness_with_alias_attribute
     Topic.alias_attribute :new_title, :title
     Topic.validates_uniqueness_of(:new_title)