Fix polymorphic eager load with foreign_key as String.

The foreign_key could be `String` and just doing `owners_map[owner_key]`
could return `nil`.
To prevent this bug, we should `to_s` both keys if their types are
different.

Fixes #14734.

3 files changed, 19 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index dc7314b450..9c92dc1141 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -570,6 +570,19 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert companies(:first_client).readonly_firm.readonly?
   end
 
+  def test_test_polymorphic_assignment_foreign_key_type_string
+    comment = Comment.first
+    comment.author   = Author.first
+    comment.resource = Member.first
+    comment.save
+
+    assert_equal Comment.all.to_a,
+      Comment.includes(:author).to_a
+
+    assert_equal Comment.all.to_a,
+      Comment.includes(:resource).to_a
+  end
+
   def test_polymorphic_assignment_foreign_type_field_updating
     # should update when assigning a saved record
     sponsor = Sponsor.new
diff --git a/activerecord/test/models/comment.rb b/activerecord/test/models/comment.rb
index ea261ac9ec..3cb32d9346 100644
--- a/activerecord/test/models/comment.rb
+++ b/activerecord/test/models/comment.rb
@@ -7,6 +7,9 @@ class Comment < ActiveRecord::Base
   scope :created, -> { all }
 
   belongs_to :post, :counter_cache => true
+  belongs_to :author,   polymorphic: true
+  belongs_to :resource, polymorphic: true
+
   has_many :ratings
 
   belongs_to :first_post, :foreign_key => :post_id
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 9e511af3bd..67ba358843 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -187,6 +187,9 @@ ActiveRecord::Schema.define do
     t.integer :taggings_count, default: 0
     t.integer :children_count, default: 0
     t.integer :parent_id
+    t.references :author, polymorphic: true
+    t.string :resource_id
+    t.string :resource_type
   end
 
   create_table :companies, force: true do |t|