Rails - Aug 2012 - Polymorphic one-to-one associations in Rails 3.2 - what's the "correct" Rails way?

Folks,

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).  New to Rails so 
please forgive my ignorance. The situation is simple: I have the
following models/tables

- Person
generic concept (name, age, nationality, etc) which I want to attach 
polymorphically to concrete models suchs

- Staff
- Mother
- Farther
- Child
- etc

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.

If it matters this is for a charity.

Regards, Neil

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On 1 August 2012 02:17, Neil Mowbray
<neil.timothy.mowbray-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
> 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

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Hello HTH,

Thanks. I must have missed in the doc as I thought one could not put 
polymorphic => true on has_one.

Regarding a person having multiple roles (Staff + Mother) I noticed the 
issue and was planning on putting
an intermediate Peron -> Role on-to-many in but you way might be easier.

Regards, Neil

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