Fix multi joins in resource and refactor methods to be pseudo private.

Author: aldhsu

lib/jsonapi/resource.rb

@@ -534,8 +534,8 @@ def apply_sort(records, order_options, _context = {})
             if field.to_s.include?(".")
               *model_names, column_name = field.split(".")
 
-              associations = lookup_association_chain([records.model.to_s, *model_names])
-              joins_query = build_joins([records.model, *associations])
+              associations = _lookup_association_chain([records.model.to_s, *model_names])
+              joins_query = _build_joins([records.model, *associations])
 
               order_by_query = "#{associations.last.name}_sorting.#{column_name} #{direction}"
               records = records.joins(joins_query).order(order_by_query)
@@ -548,28 +548,27 @@ def apply_sort(records, order_options, _context = {})
         records
       end
 
-      def lookup_association_chain(model_names)
+      def _lookup_association_chain(model_names)
         associations = []
         model_names.inject do |prev, current|
-          p = prev.classify.constantize.reflect_on_all_associations.detect do |assoc|
+          association = prev.classify.constantize.reflect_on_all_associations.detect do |assoc|
             assoc.name.to_s.downcase == current.downcase
           end
-          associations << p
-          p.class_name
+          associations << association
+          association.class_name
         end
 
         associations
       end
 
-      def build_joins(associations)
-        joins = ""
+      def _build_joins(associations)
+        joins = []
 
         associations.inject do |prev, current|
           joins << "LEFT JOIN #{current.table_name} AS #{current.name}_sorting ON #{current.name}_sorting.id = #{prev.table_name}.#{current.foreign_key}"
           current
         end
-
-        joins
+        joins.join("\n")
       end
 
       def apply_filter(records, filter, value, options = {})

test/controllers/controller_test.rb

@@ -302,20 +302,31 @@ def test_sorting_by_multiple_fields
     assert_equal '14', json_response['data'][0]['id']
   end
 
+  def create_alphabetically_first_user_and_post
+    author = Person.create(name: "Aardvark", date_joined: Time.now)
+    author.posts.create(title: "My first post", body: "Hello World")
+  end
+
   def test_sorting_by_relationship_field
-    get :index, {sort: 'author.name'}
+    post  = create_alphabetically_first_user_and_post
+    get :index, params: {sort: 'author.name'}
 
     assert_response :success
-    assert json_response['data'].length > 10
-    assert_equal '17', json_response['data'][0]['id']
+    assert json_response['data'].length > 10, 'there are enough recordsto show sort'
+    assert_equal '17', json_response['data'][0]['id'], 'nil is at the top'
+    assert_equal post.id.to_s, json_response['data'][1]['id'], 'alphabetically first user is second'
   end
 
   def test_desc_sorting_by_relationship_field
+    post  = create_alphabetically_first_user_and_post
     get :index, {sort: '-author.name'}
 
     assert_response :success
-    assert json_response['data'].length > 10
-    assert_equal '17', json_response['data'][-1]['id']
+    assert json_response['data'].length > 10, 'there are enough records to show sort'
+    assert_equal '17', json_response['data'][-1]['id'], 'nil is at the bottom'
+    assert_equal post.id.to_s, json_response['data'][-2]['id'], 'alphabetically first user is second last'
+  end
+
   end
 
   def test_invalid_sort_param

test/unit/resource/resource_test.rb

@@ -342,6 +342,31 @@ def apply_sort(records, criteria, context = {})
     end
   end
 
+  def test_lookup_association_chain
+    model_names = %w(person posts parent_post)
+    result = PostResource._lookup_association_chain(model_names)
+    assert_equal 2, result.length
+
+    posts_reflection, parent_post_reflection = result
+    assert_equal :posts, posts_reflection.name
+    assert_equal :parent_post, parent_post_reflection.name
+
+    assert_equal "posts", posts_reflection.table_name
+    assert_equal "posts", parent_post_reflection.table_name
+
+    assert_equal "author_id", posts_reflection.foreign_key
+    assert_equal "parent_post_id", parent_post_reflection.foreign_key
+  end
+
+  def test_build_joins
+    model_names = %w(person posts parent_post author)
+    associations = PostResource._lookup_association_chain(model_names)
+    result = PostResource._build_joins(associations)
+
+    assert_equal "LEFT JOIN posts AS parent_post_sorting ON parent_post_sorting.id = posts.parent_post_id
+LEFT JOIN people AS author_sorting ON author_sorting.id = posts.author_id", result
+  end
+
   def test_to_many_relationship_pagination
     post_resource = PostResource.new(Post.find(1), nil)
     comments = post_resource.comments