verify if user relationship exists before add or remove

app/repositories/orient_db/institution_repository.rb

@@ -27,12 +27,17 @@ module OrientDb
     end
 
     def add_user(user, institution)
-      create_edge "BelongsTo", user.rid, institution.id
+      unless edge_exists? "BelongsTo", user.rid, institution.id
+        create_edge "BelongsTo", user.rid, institution.id
+      end
+
       institution
     end
 
     def remove_user(user, institution)
-      destroy_edge "BelongsTo", user.rid, institution.id
+      if edge_exists? "BelongsTo", user.rid, institution.id
+        destroy_edge "BelongsTo", user.rid, institution.id
+      end
     end
 
     def build_object(args={})