obj.id seems fairly common and also seems to fall within the range of something an object could know about itself. I find myself asking why should my object know its own id?

It doesn't seem to have a reason to have it? One of the main reason for it existing is retrieve it, and so my repositories need to know it, and thus use it for database interaction.

I also once encountered a problem where I wanted to serialize an object to JSON for a RESTful API where the id did not seem to fit in the payload, but only the URI and including it in the object made that more difficult.

Should an object know it's own id? why or why not?