Allow attribute aliases for `validates_uniqueness_of`

For now, the target attribute allows attribute aliases, but `:scope`'s
attribute does not, the cause is that the former use
`read_attribute_for_validation`, but the latter does not.

Unfortunately we cannot use `read_attribute_for_validation` in this
case, it intentionally bypass custom attribute getter to allow #7072.

To work both alias and #7072, `read_attribute` should be used to resolve
attribute aliases.

activerecord/lib/active_record/validations/uniqueness.rb

@@ -78,7 +78,7 @@ module ActiveRecord
           scope_value = if record.class._reflect_on_association(scope_item)
             record.association(scope_item).reader
           else
-            record._read_attribute(scope_item)
+            record.read_attribute(scope_item)
           end
           relation = relation.where(scope_item => scope_value)
         end

activerecord/test/cases/validations/uniqueness_validation_test.rb

@@ -156,6 +156,25 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert r3.valid?, "Saving r3"
   end
 
+  def test_validate_uniqueness_with_aliases
+    Reply.validates_uniqueness_of(:new_content, scope: :new_parent_id)
+
+    t = Topic.create(title: "I'm unique!")
+
+    r1 = t.replies.create(title: "r1", content: "hello world")
+    assert_predicate r1, :valid?, "Saving r1"
+
+    r2 = t.replies.create(title: "r2", content: "hello world")
+    assert_not_predicate r2, :valid?, "Saving r2 first time"
+
+    r2.content = "something else"
+    assert r2.save, "Saving r2 second time"
+
+    t2 = Topic.create("title" => "I'm unique too!")
+    r3 = t2.replies.create(title: "r3", content: "hello world")
+    assert_predicate r3, :valid?, "Saving r3"
+  end
+
   def test_validate_uniqueness_with_scope_invalid_syntax
     error = assert_raises(ArgumentError) do
       Reply.validates_uniqueness_of(:content, scope: { parent_id: false })

activerecord/test/models/reply.rb

@@ -10,6 +10,9 @@ class Reply < Topic
 
   scope :ordered, -> { Reply.order(:id) }
 
+  alias_attribute :new_content, :content
+  alias_attribute :new_parent_id, :parent_id
+
   # Method on Kernel
   def self.open
     approved