Help with Delete

I have this code in rp_parts controller in def destroy:

    @repair_ticket.rp_parts.delete(@rp_part)

repair_tickets and rp_parts are HABTM.

If I do in the console:

repair_ticket1.rp_parts

I get:
=> [#<RpPart id: 1, ...">, #<RpPart id: 1,...">, #<RpPart ...">]

I only want to delete one of those RpPart id:1, and not both of them. Is
it possible?