Alters build_joins sql for has_one relationships

The existing tests are passing because they seem to all use a self-
referential `has_one` relationship. If the foreign_key is on a different
table, however, the queries will fail. So if it the relationship is a
has_one, we need to alter the query to look for the foreign-key on the
related table.

Author: Ross-Hunter

lib/jsonapi/active_relation_resource_finder.rb

@@ -423,7 +423,12 @@ 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}"
+          if current.has_one?
+            joins << "LEFT JOIN #{current.table_name} AS #{current.name}_sorting ON #{current.name}_sorting.#{current.foreign_key} = #{prev.table_name}.id"
+          else
+            joins << "LEFT JOIN #{current.table_name} AS #{current.name}_sorting ON #{current.name}_sorting.id = #{prev.table_name}.#{current.foreign_key}"
+          end
+
           current
         end
         joins.join("\n")

test/unit/resource/resource_test.rb

@@ -360,12 +360,15 @@ def test_lookup_association_chain
   end
 
   def test_build_joins
-    model_names = %w(person posts parent_post author)
+    model_names = %w(person posts parent_post author author_detail)
     associations = PostResource._lookup_association_chain(model_names)
     result = PostResource.send(:_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
+    sql = "LEFT JOIN posts AS parent_post_sorting ON parent_post_sorting.parent_post_id = posts.id
+LEFT JOIN people AS author_sorting ON author_sorting.id = posts.author_id
+LEFT JOIN author_details AS author_detail_sorting ON author_detail_sorting.person_id = people.id"
+
+    assert_equal sql, result
   end
 
   # ToDo: Implement relationship pagination