adding @josenavas comment

Author: antgonza

qiita_db/study.py

@@ -394,6 +394,15 @@ def delete(cls, id_):
             "DELETE FROM qiita.study_environmental_package WHERE study_id = "
             "%s", (id_, ))
 
+        conn_handler.add_to_queue(
+            queue,
+            "DELETE FROM qiita.study_users WHERE study_id = %s", (id_, ))
+
+        conn_handler.add_to_queue(
+            queue,
+            "DELETE FROM qiita.investigation_study WHERE study_id = "
+            "%s", (id_, ))
+
         conn_handler.add_to_queue(
             queue,
             "DELETE FROM qiita.study WHERE study_id = %s", (id_, ))