Sorry Sir, I need to see your Id.

I just bumped into the following little “bug” in my rails app. I was getting the id of a related object to use to fill a newly added column in a migration, and suddenly far too many of the objects were linked to id=4. (Take particular note of the final line).

So what’s going on? It turns out that as well as a database “id” (if the object is stored in the database), every object has an object_id (which is also accessible through .id if the object is not an ActiveRecord model).

So when we do: we are actually getting the id of the nil object (since jonny.parent returns nil). This id is always 4, hence the confusion.

By doing: jonny.parent_id instead, we don’t try and actually retrieve the parent object, but just look at the database row to get the value.

Admittedly you will get a warning if you do : warning: Object#id will be deprecated; use Object#object_id, not that I spotted the warning.

So maybe this issue will disappear in the future, if .id gets removed from the Ruby syntax, but until then consider yourself warned.