diff --git a/lib/jsonapi/deserialization.rb b/lib/jsonapi/deserialization.rb
index 0a9d5de..3d97c7b 100644
--- a/lib/jsonapi/deserialization.rb
+++ b/lib/jsonapi/deserialization.rb
@@ -62,7 +62,7 @@ def jsonapi_deserialize(document, options = {})
 
       relationships.map do |assoc_name, assoc_data|
         assoc_data = (assoc_data || {})['data'] || {}
-        rel_name = jsonapi_inflector.singularize(assoc_name)
+        rel_name = jsonapi_inflector.singularize(assoc_name).underscore
 
         if assoc_data.is_a?(Array)
           parsed["#{rel_name}_ids"] = assoc_data.map { |ri| ri['id'] }.compact