Generate secure token only once regardless of `on: :initialize` or `on: :create`

Follow-up to #47420.

Whereas the original behavior (`on: :create`) is invoked only once
before a record is persisted, the new behavior (`on: :initialize`) is
invoked not only new record but also persisted records.

It should be invoked only once for new record consistently.

activerecord/lib/active_record/secure_token.rb

@@ -52,7 +52,9 @@ module ActiveRecord
         require "active_support/core_ext/securerandom"
         define_method("regenerate_#{attribute}") { update! attribute => self.class.generate_unique_secure_token(length: length) }
         set_callback on, on == :initialize ? :after : :before do
-          send("#{attribute}=", self.class.generate_unique_secure_token(length: length)) if has_attribute?(attribute) && !send("#{attribute}?")
+          if new_record? && !query_attribute(attribute)
+            write_attribute(attribute, self.class.generate_unique_secure_token(length: length))
+          end
         end
       end
 

activerecord/test/cases/secure_token_test.rb

@@ -25,6 +25,22 @@ class SecureTokenTest < ActiveRecord::TestCase
     assert_equal token, User.find(@user.id).token
   end
 
+  def test_generating_token_on_initialize_happens_only_once
+    model = Class.new(ActiveRecord::Base) do
+      self.table_name = "users"
+      has_secure_token on: :initialize
+    end
+
+    token = "    "
+
+    user = model.new
+    user.update!(token: token)
+
+    assert_equal token, user.token
+    assert_equal token, user.reload.token
+    assert_equal token, model.find(user.id).token
+  end
+
   def test_generating_token_on_initialize_is_skipped_if_column_was_not_selected
     model = Class.new(ActiveRecord::Base) do
       self.table_name = "users"