Fix AR signed IDs for STI models

Closes #40187.

activerecord/lib/active_record/signed_id.rb

@@ -90,7 +90,7 @@ def signed_id_verifier=(verifier)
 
       # :nodoc:
       def combine_signed_id_purposes(purpose)
-        [ name.underscore, purpose.to_s ].compact_blank.join("/")
+        [ base_class.name.underscore, purpose.to_s ].compact_blank.join("/")
       end
     end
 

activerecord/test/cases/signed_id_test.rb

@@ -11,7 +11,7 @@
 ActiveRecord::Base.signed_id_verifier_secret = SIGNED_ID_VERIFIER_TEST_SECRET
 
 class SignedIdTest < ActiveRecord::TestCase
-  fixtures :accounts, :toys
+  fixtures :accounts, :toys, :companies
 
   setup do
     @account = Account.first
@@ -26,6 +26,10 @@ class SignedIdTest < ActiveRecord::TestCase
     assert_equal @toy, Toy.find_signed(@toy.signed_id)
   end
 
+  test "find signed record for single table inheritance (STI Models)" do
+    assert_equal Company.first, Company.find_signed(Company.first.signed_id)
+  end
+
   test "raise UnknownPrimaryKey when model have no primary key" do
     error = assert_raises(ActiveRecord::UnknownPrimaryKey) do
       Matey.find_signed("this will not be even verified")