Eager evaluate relation in composite case

Normally, it's valid syntax to pass the predicate builder a mapping from
primary key to a record, to an id, or to a relation.

With the composite case, there's a limitation on the types of syntax
supported. Namely, the only case we support right now is mapping a tuple
of columns to a tuple of corresponding values. For now, it's sufficient
to extract the ids instead of evaluating the SQL at a later time.

activerecord/lib/active_record/relation/predicate_builder/association_query_value.rb

@@ -10,7 +10,10 @@ def initialize(associated_table, value)
 
       def queries
         if associated_table.join_foreign_key.is_a?(Array)
-          ids.map { |ids_set| associated_table.join_foreign_key.zip(ids_set).to_h }
+          id_list = ids
+          id_list = id_list.pluck(primary_key) if id_list.is_a?(Relation)
+
+          id_list.map { |ids_set| associated_table.join_foreign_key.zip(ids_set).to_h }
         else
           [ associated_table.join_foreign_key => ids ]
         end

activerecord/test/cases/associations_test.rb

@@ -175,6 +175,14 @@ def test_querying_by_single_associated_record_works_using_query_constraints
     assert_equal(expected_posts.map(&:id).sort, blog_posts.map(&:id).sort)
   end
 
+  def test_querying_by_relation_with_composite_key
+    expected_posts = [sharded_blog_posts(:great_post_blog_one), sharded_blog_posts(:great_post_blog_two)]
+
+    blog_posts = Sharded::BlogPost.where(comments: Sharded::Comment.where(body: "I really enjoyed the post!")).to_a
+
+    assert_equal(expected_posts.map(&:id).sort, blog_posts.map(&:id).sort)
+  end
+
   def test_has_many_association_with_composite_foreign_key_loads_records
     blog_post = sharded_blog_posts(:great_post_blog_one)