I’ve gotten this error so many times while building Rails applications and it didn’t make any sense. It usually appeared when I was trying to call Some_Object.id and the object wasn’t assigned correctly, so in essence I was calling .id on nil. But I had no idea why that would “mistakenly be 4”.

Thanks to Tim, I finally unraveled the mystery.

The first important thing to know is that in Ruby, everything is an object. No really, everything. Even the class “Object” which contains everything inside it is an object. Think of it like an index of keywords, and each one has an object_id. The object_id for the Object class is 18703980.

This index goes like this:

All odd indexes are integers. E.g. 0.object_id is 1, 1.object_id is 3, 2.object_id is 5, etc. All immutable objects always have the same object_id (Classes, Symbols, Fixnum, etc). Mutable objects like strings DON’T have the same object id. E.g. “apple”.object_id != “apple”.object_id. I’m sure there are more rules, but these are the ones I’ve discovered so far.

Getting back to nil

There’s no particular reason for this that I can see, but Ruby was designed so that the first object in this index is false. So, false.object_id = 0. The next non-integer object is true. And finally, the third non-integer object is nil.

1.9.3p125 :050 > false.object_id
 => 0
1.9.3p125 :051 > true.object_id
 => 2
1.9.3p125 :052 > nil.object_id
 => 4

My next question was obviously, how do I find what object is assigned to a given object_id. A quick search revealed that

ObjectSpace._id2ref(object_id)

will return the object associated with the given parameter.

Oh and, by the way. the ObjectSpace class also has an object_id. :)