`:polymorphic`, `:as`, and `:foreign_type` are valid for polymorphic association

activerecord/lib/active_record/associations/builder/belongs_to.rb

@@ -7,7 +7,9 @@ module ActiveRecord::Associations::Builder # :nodoc:
     end
 
     def self.valid_options(options)
-      super + [:polymorphic, :counter_cache, :optional, :default]
+      valid = super + [:counter_cache, :optional, :default]
+      valid += [:polymorphic, :foreign_type] if options[:polymorphic]
+      valid
     end
 
     def self.valid_dependent_options

activerecord/lib/active_record/associations/builder/has_many.rb

@@ -7,7 +7,8 @@ module ActiveRecord::Associations::Builder # :nodoc:
     end
 
     def self.valid_options(options)
-      valid = super + [:as, :counter_cache, :join_table, :foreign_type, :index_errors]
+      valid = super + [:counter_cache, :join_table, :index_errors]
+      valid += [:as, :foreign_type] if options[:as]
       valid += [:through, :source, :source_type] if options[:through]
       valid
     end

activerecord/lib/active_record/associations/builder/has_one.rb

@@ -7,7 +7,8 @@ module ActiveRecord::Associations::Builder # :nodoc:
     end
 
     def self.valid_options(options)
-      valid = super + [:as]
+      valid = super
+      valid += [:as, :foreign_type] if options[:as]
       valid += [:through, :source, :source_type] if options[:through]
       valid
     end

activerecord/lib/active_record/associations/builder/singular_association.rb

@@ -5,7 +5,7 @@
 module ActiveRecord::Associations::Builder # :nodoc:
   class SingularAssociation < Association #:nodoc:
     def self.valid_options(options)
-      super + [:foreign_type, :required, :touch]
+      super + [:required, :touch]
     end
 
     def self.define_accessors(model, reflection)

activerecord/test/cases/associations/has_many_associations_test.rb

@@ -2439,6 +2439,7 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     post.images << image
 
     assert_equal [image], post.images
+    assert_equal post, image.imageable
   end
 
   def test_build_with_polymorphic_has_many_does_not_allow_to_override_type_and_id

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -691,6 +691,7 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
     post.reload
 
     assert_equal image, post.main_image
+    assert_equal post, image.imageable
   end
 
   test "dangerous association name raises ArgumentError" do

activerecord/test/models/image.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 class Image < ActiveRecord::Base
-  belongs_to :imageable, foreign_key: :imageable_identifier, foreign_type: :imageable_class
+  belongs_to :imageable, polymorphic: true, foreign_key: :imageable_identifier, foreign_type: :imageable_class
 end