Repeater basics

I'm sorry guys, I feel dumb asking. Maybe I'm over tired but I've been racking my brains in the docs and just can't see this simple thing. I have a to_do model and in that model is a nullable invoice_id. So each task can be optionally attached to an invoice. In my invoice edit form I collect some other data but I want to attach multiple to_dos. I thought I'd use a repeater but it's trying to CREATE a new to_do. I just want to attach multiple to_dos that already exist to this new invoice I'm creating. Can I not use a repeater for this? Cause this doesn't work. It tries to create a to_do.