Allow querying by whole record for composite query constraints associations

Given an association defined with composite query constraints like:
```ruby
BlogPost.has_many :comments, query_constraints: [:blog_id, :blog_post_id]
```

it is possible to query blog posts by whole `comments` objects like:

```ruby
comments = Comment.first(2)
BlogPost.where(comments: comments).to_a
```

Author: nvasilevski

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

@@ -9,7 +9,11 @@ def initialize(associated_table, value)
       end
 
       def queries
-        [ associated_table.join_foreign_key => ids ]
+        if associated_table.join_foreign_key.is_a?(Array)
+          ids.map { |ids_set| associated_table.join_foreign_key.zip(ids_set).to_h }
+        else
+          [ associated_table.join_foreign_key => ids ]
+        end
       end
 
       private
@@ -31,6 +35,8 @@ def primary_key
         end
 
         def convert_to_id(value)
+          return primary_key.map { |pk| value.public_send(pk) } if primary_key.is_a?(Array)
+
           if value.respond_to?(primary_key)
             value.public_send(primary_key)
           else

activerecord/test/cases/associations_test.rb

@@ -147,6 +147,15 @@ def test_belongs_to_a_model_with_composite_primary_key_uses_composite_pk_in_sql
     assert_match(/#{Regexp.escape(Sharded::BlogPost.connection.quote_table_name("sharded_blog_posts.id"))} =/, sql)
   end
 
+  def test_querying_by_whole_associated_records_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).to_a
+
+    expected_posts = [sharded_blog_posts(:great_post_blog_one), 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)
 

activerecord/test/fixtures/sharded_comments.yml

@@ -18,5 +18,5 @@ unique_comment_blog_post_one:
 
 great_comment_blog_post_two:
   body: "I really enjoyed the post!"
-  blog_post_id: <%= ActiveRecord::FixtureSet.identify(:great_blog_post_two) %>
+  blog_post_id: <%= ActiveRecord::FixtureSet.identify(:great_post_blog_two) %>
   blog_id: <%= ActiveRecord::FixtureSet.identify(:sharded_blog_two) %>