Define a class method to introspect valid delegatable types so they can

be used to validate the type column and power strong parameters.

Author: JP Rosevear

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Add `<role>_types` class method to `ActiveRecord::DelegatedType` so that the delegated types can be instrospected
+
+    *JP Rosevear*
+
 *   Make `ActiveRecord::Encryption::Encryptor` agnostic of the serialization format used for encrypted data.
 
     Previously, the encryptor instance only allowed an encrypted value serialized as a `String` to be passed to the message serializer.

activerecord/lib/active_record/delegated_type.rb

@@ -219,6 +219,10 @@ def define_delegated_type_methods(role, types:, options:)
         role_type = options[:foreign_type] || "#{role}_type"
         role_id   = options[:foreign_key] || "#{role}_id"
 
+        define_singleton_method "#{role}_types" do
+          types.map(&:to_s)
+        end
+
         define_method "#{role}_class" do
           public_send(role_type).constantize
         end

activerecord/test/cases/delegated_type_test.rb

@@ -24,6 +24,10 @@ class DelegatedTypeTest < ActiveRecord::TestCase
     end
   end
 
+  test "delegated types" do
+    assert_equal ["Message", "Comment"], Entry.entryable_types
+  end
+
   test "delegated class" do
     assert_equal Message, @entry_with_message.entryable_class
     assert_equal Comment, @entry_with_comment.entryable_class

guides/source/association_basics.md

@@ -3007,6 +3007,7 @@ With this definition complete, our `Entry` delegator now provides the following
 
 | Method | Return |
 |---|---|
+| `Entry#entryable_types` | ["Message", "Comment"] |
 | `Entry#entryable_class` | Message or Comment |
 | `Entry#entryable_name` | "message" or "comment" |
 | `Entry.messages` | `Entry.where(entryable_type: "Message")` |