Backward compatibility to work Marshal.load(legacy_relation.dump)

A relation has a predicate builder, so if a predicate handler which is
referenced by a legacy predicate builder is removed in the new code,
`Marshal.load(legacy_relation.dump)` will fail due to referencing
undefined constant.

I've restored no-op `BaseHandler` constant alias to make cache rotation
easier during Rails 6.1.

Fixes #39601.

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -2,6 +2,17 @@
 
 module ActiveRecord
   class PredicateBuilder # :nodoc:
+    require "active_record/relation/predicate_builder/array_handler"
+    require "active_record/relation/predicate_builder/basic_object_handler"
+    require "active_record/relation/predicate_builder/range_handler"
+    require "active_record/relation/predicate_builder/relation_handler"
+    require "active_record/relation/predicate_builder/association_query_value"
+    require "active_record/relation/predicate_builder/polymorphic_array_value"
+
+    # No-op BaseHandler to work Mashal.load(File.read("legacy_relation.dump")).
+    # TODO: Remove the constant alias once Rails 6.1 has released.
+    BaseHandler = BasicObjectHandler
+
     def initialize(table)
       @table = table
       @handlers = []
@@ -156,11 +167,3 @@ module ActiveRecord
       end
   end
 end
-
-require "active_record/relation/predicate_builder/array_handler"
-require "active_record/relation/predicate_builder/basic_object_handler"
-require "active_record/relation/predicate_builder/range_handler"
-require "active_record/relation/predicate_builder/relation_handler"
-
-require "active_record/relation/predicate_builder/association_query_value"
-require "active_record/relation/predicate_builder/polymorphic_array_value"

activerecord/test/cases/relation_test.rb

@@ -418,6 +418,14 @@ module ActiveRecord
       end
     end
 
+    def test_marshal_load_legacy_relation
+      path = File.expand_path(
+        "support/marshal_compatibility_fixtures/legacy_relation.dump",
+        TEST_ROOT
+      )
+      assert_equal 11, Marshal.load(File.read(path)).size
+    end
+
     test "no queries on empty IN" do
       assert_queries(0) do
         Post.where(id: []).load

activerecord/test/cases/yaml_serialization_test.rb

@@ -132,8 +132,8 @@ class YamlSerializationTest < ActiveRecord::TestCase
   private
     def yaml_fixture(file_name)
       path = File.expand_path(
-        "../support/yaml_compatibility_fixtures/#{file_name}.yml",
-        __dir__
+        "support/yaml_compatibility_fixtures/#{file_name}.yml",
+        TEST_ROOT
       )
       File.read(path)
     end