[PhilMeyr]

Motivation

A has_many/has_one :through association whose source is a polymorphic belongs_to is generated as untyped, even when source_type: pins the class:

class Client < ApplicationRecord
  has_many :safe_keeper_abilities
  has_many :safe_keepers, through: :safe_keeper_abilities, source: :resource, source_type: "SafeKeeper"
end

sig { returns(ActiveRecord::Associations::CollectionProxy) }
def safe_keepers; end

Rails resolves the class from source_type (Client.reflect_on_association(:safe_keepers).klass # => SafeKeeper), so the association can only hold SafeKeeper records. polymorphic_association? follows the source reflection, which is polymorphic, and ignores source_type.

#1990 fixed the same symptom by dropping the polymorphic check entirely, and was closed with a CI failure. This change is narrower: only a through reflection with source_type is treated as non-polymorphic. A polymorphic belongs_to and a through association without source_type are unchanged.

Implementation

polymorphic_association? returns false for a through reflection whose options include source_type. type_for and relation_type_for then use reflection.klass, which Rails computes from source_type.

Tests

Added a spec, with and without relations enabled, in spec/tapioca/dsl/compilers/active_record_associations_spec.rb:

• has_many :through with source_type → ::SafeKeeper::PrivateCollectionProxy (::ActiveRecord::Associations::CollectionProxy[::SafeKeeper] without relations);
• has_one :through with source_type → T.nilable(::SafeKeeper), and typed build_/create_/reload_ methods.

Both fail without the change.

[PhilMeyr]

I have signed the CLA!

[amomchilov]

Thanks!