Fix cache_belongs_to prefetch with owners sharing a foreign key

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Unreleased
 
+- Fix `cache_belongs_to` prefetch assigning the record to only one of several owners that share a foreign key.
+
 ## 1.6.5
 
 - Improve MemCacheStoreCAS patch to properly pass serialized entries to Dalli.

lib/identity_cache/cached/belongs_to.rb

@@ -70,21 +70,22 @@ def fetch_async(load_strategy, records)
             yield batch_records
           end
         else
-          ids_to_owner_record = records.each_with_object({}) do |owner_record, hash|
+          ids_to_owner_records = records.each_with_object(Hash.new { |h, k| h[k] = [] }) do |owner_record, hash|
             associated_id = owner_record.send(reflection.foreign_key)
             if associated_id && !owner_record.instance_variable_defined?(records_variable_name)
-              hash[associated_id] = owner_record
+              hash[associated_id] << owner_record
             end
           end
 
-          if ids_to_owner_record.any?
+          if ids_to_owner_records.any?
             load_strategy.load_multi(
               reflection.klass.cached_primary_index,
-              ids_to_owner_record.keys
+              ids_to_owner_records.keys
             ) do |associated_records_by_id|
               associated_records_by_id.each do |id, associated_record|
-                owner_record = ids_to_owner_record.fetch(id)
-                write(owner_record, associated_record)
+                ids_to_owner_records.fetch(id).each do |owner_record|
+                  write(owner_record, associated_record)
+                end
               end
 
               yield associated_records_by_id.values.compact

test/prefetch_associations_test.rb

@@ -100,6 +100,24 @@ def test_prefetch_associations_cached_belongs_to
       end
     end
 
+    def test_prefetch_associations_cached_belongs_to_with_shared_foreign_key
+      Item.send(:cache_belongs_to, :item)
+      @bob.update!(item_id: @fred.id)
+      @joe.update!(item_id: @fred.id)
+      @bob.fetch_item
+      @joe.fetch_item
+      items = [@bob, @joe].map(&:reload)
+
+      assert_no_queries do
+        assert_memcache_operations(1) do
+          prefetch(Item, :item, items)
+        end
+        assert_memcache_operations(0) do
+          assert_equal([@fred, @fred], items.map(&:fetch_item))
+        end
+      end
+    end
+
     def test_prefetch_associations_notifies_about_hydration
       Item.send(:cache_belongs_to, :item)
       @bob.update!(item_id: @joe.id)
@@ -346,6 +364,29 @@ def test_fetch_multi_batch_fetches_non_embedded_second_level_belongs_to_associat
       end
     end
 
+    def test_fetch_multi_batch_fetches_non_embedded_second_level_belongs_to_associations_with_shared_foreign_key
+      Item.send(:cache_has_many, :associated_records, embed: :ids)
+      AssociatedRecord.send(:cache_belongs_to, :item_two)
+
+      shared_item_two = ItemTwo.create!(title: "shared")
+      child_records = [@bob, @bob, @joe].map do |parent|
+        parent.associated_records.create!(item_two: shared_item_two)
+      end
+
+      # populate the cache entries and associated children ID variables
+      ItemTwo.fetch(shared_item_two.id)
+      AssociatedRecord.fetch_multi(child_records.map(&:id))
+      Item.fetch_multi(@bob.id, @joe.id)
+
+      assert_memcache_operations(3) do
+        cached_bob, cached_joe = Item.fetch_multi(
+          @bob.id, @joe.id, includes: { associated_records: :item_two }
+        )
+        cached_child_records = cached_bob.fetch_associated_records + cached_joe.fetch_associated_records
+        assert_equal([shared_item_two] * 3, cached_child_records.map(&:fetch_item_two))
+      end
+    end
+
     def test_fetch_multi_doesnt_batch_fetches_belongs_to_associations_if_the_foreign_key_isnt_present
       AssociatedRecord.send(:cache_belongs_to, :item)
       @child = AssociatedRecord.create!(name: "bob child")