has_only_one or has_mate

has_one means that the entity has 0 or 1. It is a “has only one.”

It looks like you’re trying a self-referencing relationship. Make sure the Person (assuming) model has both has_one and belongs_to methods.

Something like:

has_one :friend, :class => ‘Person’
belongs_to :person

Oops…

has_one :friend, :class => ‘Person’
belongs_to :person, :foreign_key => ‘friend’