Added option to validate which models are allowed to be associated to a polymorphic belongs_to

activerecord/CHANGELOG.md

@@ -1,3 +1,13 @@
+*   Added option to validate which models are allowed to be associated to a polymorphic `belongs_to`.
+
+    Ex:
+    ```ruby
+    belongs_to :commentable, polymorphic: ["Post", "Comment"]
+    ```
+    This automatically adds an inclusion validator to commentable_type, to ensure that it's either Post or Comment.
+
+    *Nate Matykiewicz*
+
 *   Improve `ActiveRecord::Store` to raise a descriptive exception if the column is not either
     structured (e.g., PostgreSQL +hstore+/+json+, or MySQL +json+) or declared serializable via
     `ActiveRecord.store`.

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

@@ -121,6 +121,12 @@ def self.define_validations(model, reflection)
         required = !reflection.options[:optional]
       end
 
+      if reflection.options[:polymorphic].is_a?(Array)
+        model.validates reflection.foreign_type,
+          inclusion: { in: reflection.options[:polymorphic].map { |klass| klass.to_s.freeze } },
+          if: ->(record) { record.read_attribute(reflection.foreign_type) }
+      end
+
       super
 
       if required

activerecord/test/cases/associations/belongs_to_associations_test.rb

@@ -1374,6 +1374,28 @@ def test_polymorphic_counter_cache
     end
   end
 
+  class TestPolymorphicArrayAllowed < ActiveRecord::Base
+    self.table_name = "wheels"
+    belongs_to :wheelable, polymorphic: ["Book", :Essay], optional: false
+  end
+
+  def test_polymorphic_with_allowed_class_string_array
+    wheel = TestPolymorphicArrayAllowed.new
+
+    assert_not_predicate wheel, :valid?
+    assert_equal ["Wheelable must exist"], wheel.errors.full_messages
+
+    wheel.wheelable = Book.create!
+    assert_predicate wheel, :valid?
+
+    wheel.wheelable = Essay.create!
+    assert_predicate wheel, :valid?
+
+    wheel.wheelable = Citation.create!
+    assert_not_predicate wheel, :valid?
+    assert_equal ["Wheelable type is not included in the list"], wheel.errors.full_messages
+  end
+
   def test_polymorphic_with_custom_foreign_type
     sponsor = sponsors(:moustache_club_sponsor_for_groucho)
     groucho = members(:groucho)

guides/source/association_basics.md

@@ -538,6 +538,16 @@ class CreatePictures < ActiveRecord::Migration[7.2]
 end
 ```
 
+In this example, `imageable` can have any model assigned to it. If you wanted to ensure that the `imageable_type` is only `Employee` or `Product`, you can set the `polymorphic` option to an array of allowed class names. This will [add an inclusion validator][inclusion validator] to the `imageable_type` column, which adds a validation error if `imageable` is not one of the expected classes:
+
+[inclusion validator]: active_record_validations.html#inclusion
+
+```ruby
+class Picture < ApplicationRecord
+  belongs_to :imageable, polymorphic: ["Employee", "Product"]
+end
+```
+
 NOTE: Since polymorphic associations rely on storing class names in the
 database, that data must remain synchronized with the class name used by the
 Ruby code. When renaming a class, make sure to update the data in the