CPK: Fix specifying single record in association

Single records in CPK contexts will take on an array to represent their
identifier. This was problematic because if only a single record is
passed, it's hard to distinguish the behaviour between a list of
multiple primary keys or a list representing a single primary key. As
such, it's helpful to always wrap the result of the single ID in an array,
so as to disambiguate the cases and create consistency in the shape of
the id structure.

Author: paarthmadan

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

@@ -26,7 +26,7 @@ def ids
           when Array
             value.map { |v| convert_to_id(v) }
           else
-            convert_to_id(value)
+            [convert_to_id(value)]
           end
         end
 

activerecord/test/cases/associations_test.rb

@@ -166,6 +166,15 @@ def test_querying_by_whole_associated_records_using_query_constraints
     assert_equal(expected_posts.map(&:id).sort, blog_posts.map(&:id).sort)
   end
 
+  def test_querying_by_single_associated_record_works_using_query_constraints
+    comments = [sharded_comments(:great_comment_blog_post_one), sharded_comments(:great_comment_blog_post_two)]
+
+    blog_posts = Sharded::BlogPost.where(comments: comments.last).to_a
+
+    expected_posts = [sharded_blog_posts(:great_post_blog_two)]
+    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)