Deprecate `.reorder(nil)` with `.first` / `.first!` taking non-deterministic result

To continue taking non-deterministic result, use `.take` / `.take!`
instead.

activerecord/CHANGELOG.md

+*   Deprecate `.reorder(nil)` with `.first` / `.first!` taking non-deterministic result.
+
+    To continue taking non-deterministic result, use `.take` / `.take!` instead.
+
+    *Ryuta Kamizono*
+
 *   Ensure custom PK types are casted in through reflection queries.
 
     *Gannon McGibbon*
@@ -78,4 +84,5 @@
 
     *Michael Duchemin*
 
+
 Please check [6-0-stable](https://github.com/rails/rails/blob/6-0-stable/activerecord/CHANGELOG.md) for previous changes.

activerecord/lib/active_record/relation/finder_methods.rb

@@ -114,6 +114,15 @@ def take!
     #   Person.first(3) # returns the first three objects fetched by SELECT * FROM people ORDER BY people.id LIMIT 3
     #
     def first(limit = nil)
+      if !order_values.empty? && order_values.all?(&:blank?)
+        blank_value = order_values.first
+        ActiveSupport::Deprecation.warn(<<~MSG.squish)
+          `.reorder(#{blank_value.inspect})` with `.first` / `.first!` no longer
+          takes non-deterministic result in Rails 6.2.
+          To continue taking non-deterministic result, use `.take` / `.take!` instead.
+        MSG
+      end
+
       if limit
         find_nth_with_limit(0, limit)
       else

activerecord/test/cases/relations_test.rb

@@ -1739,7 +1739,14 @@ def test_reverse_order_with_reorder_nil_removes_the_order
 
   def test_reorder_with_first
     sql_log = capture_sql do
-      assert Post.order(:title).reorder(nil).first
+      message = <<~MSG.squish
+        `.reorder(nil)` with `.first` / `.first!` no longer
+        takes non-deterministic result in Rails 6.2.
+        To continue taking non-deterministic result, use `.take` / `.take!` instead.
+      MSG
+      assert_deprecated(message) do
+        assert Post.order(:title).reorder(nil).first
+      end
     end
     assert sql_log.all? { |sql| !/order by/i.match?(sql) }, "ORDER BY was used in the query: #{sql_log}"
   end