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.

1 files changed, 17 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index bf461070e0..5a172a417d 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -58,7 +58,11 @@ module ActiveRecord
 
         def owners_by_key
           @owners_by_key ||= owners.group_by do |owner|
-            owner[owner_key_name]
+            if owner_key_type == association_key_type
+              owner[owner_key_name]
+            else
+              owner[owner_key_name].to_s
+            end
           end
         end
 
@@ -93,13 +97,24 @@ module ActiveRecord
           records_by_owner
         end
 
+        def association_key_type
+          @klass.column_types[association_key_name.to_s].type
+        end
+
+        def owner_key_type
+          @model.column_types[owner_key_name.to_s].type
+        end
+
         def load_slices(slices)
           @preloaded_records = slices.flat_map { |slice|
             records_for(slice)
           }
 
           @preloaded_records.map { |record|
-            [record, record[association_key_name]]
+            key = record[association_key_name]
+            key = key.to_s unless owner_key_type == association_key_type
+
+            [record, key]
           }
         end