Unless I'm being dumb I cannot find anything on Google search or in the Rails books on how to create a one-to-one polymorphic association (as apposed to a one-to-many polymorphic association).
Change the has_many to a has_one, and you should have a 1:1 relationship...
- Person generic concept (name, age, nationality, etc) which I want to attach polymorphically to concrete models suchs
concrete class contain additional type specific information which I do not want nullable hence STI is not desirable and would result in a sparse table.
I would still consider STI for this kind of situation, but I'd give each subclass a different association to the specific information for that model, which would keep the DB nice and 'clean' (I did it yesterday with Vehicles, and sub-classes for Car, Truck, Bus, Motorbike, etc,)
class Person < AR::Base end
class Staff < Person # employee number, start date, etc has_one :staff_details end
class Mother < Person # can't imagine what info "mother" would need, but there's support for it has_one :mother_details end
class Child < Person # what does a child need differently? has_one :child_details end
Of course, the problem with this approach is what happens when you
have a staff member who is a mother? You'd need two records for them
(it's easy for me; a Car can't also be a Motorbike 
In this event you would be better coming from a different angle (like
a "has_many :employments" to determine whether a Person is staff or
not, and a "has_many :parentings" to figure whether they're a mother
or father)
HTH