Rails - Jul 2011 - polymorphic assoc seems only best for multiple parents ?

I have done a little polymorphic associations stuff and have
refreshed my memory on it again.
What it seems like is that if I have a particular record and I want to
make it easy for many different parent records to associate with it
using has_one or has_many, that is fine.

 If I want a parent record to have multiple kinds of children through
one association, I don''t see how to do that (except maybe with STI but
then they all have to share the same fields in one record). I figure
there must be some way to do this with a kind of intermediate join or
something, but I haven''t seen any examples that I can recall ..

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On Wednesday, July 20, 2011 7:04:09 AM UTC-6, Jedrin
wrote:
>
>
>  I have done a little polymorphic associations stuff and have 
> refreshed my memory on it again. 
> What it seems like is that if I have a particular record and I want to 
> make it easy for many different parent records to associate with it 
> using has_one or has_many, that is fine. 
>
>  If I want a parent record to have multiple kinds of children through 
> one association, I don''t see how to do that (except maybe with STI
but
> then they all have to share the same fields in one record). I figure 
> there must be some way to do this with a kind of intermediate join or 
> something, but I haven''t seen any examples that I can recall ..

If I understand what you''re saying then no, it isn''t even
possible using a
join table. This is the quick example I tested with:

# The following demonstrates that this DOES NOT work...
class Region < ActiveRecord::Base
  # a.k.a the "parent" that wants to "own" several
  # different kinds of children through one relation (shapes)
  has_many :regions_shapes
  has_many :shapes, :through => :regions_shapes
end

class RegionsShape < ActiveRecord::Base
  # polymorphic join model
  belongs_to :region
  belongs_to :shape, :polymorphic => true
end

class Ellipse < ActiveRecord::Base
  # one of two types of "children" that can belong to a
  # Region (parent) through its "shapes" relation
  has_one :regions_shape, :as => :shape
  has_one :region, :through => :regions_shape
end

class Polygon < ActiveRecord::Base
  # the other of the two types of "children"
  has_one :regions_shape, :as => :shape
  has_one :region, :through => :regions_shape
end

All of the relationships work fine EXCEPT for the one you wanted, in this 
example Region#shapes. Trying to use it will raise:

ActiveRecord::HasManyThroughAssociationPolymorphicError: Cannot have a 
has_many :through association ''Region#shapes'' on the
polymorphic object
''Shape#shape''

I think you''d have to simply have several explicit relationships for
each
type that the parent can "have":

class Region < ActiveRecord::Base
  has_many :ellipses
  has_many :polygons
end

class Polygon < ActiveRecord::Base
  belongs_to :region
end

class Ellipse < ActiveRecord::Base
  belongs_to :region
end

Then just create a "combining" method for each relation-like action
that the
various relation methods support. Since you aren''t working with actual 
relation objects, you can''t chain or further filter on the results
though
so....

  def shapes
    polygons + ellipses # or something like that for an Array (not a 
relation)
  end

I''m assuming this is because if #shapes *could* be/return a relation
then
said relation would really have to have the ability to map to _n_ seperate 
SQL select statements (one for each table that can possibly be a
"child").
As such, further filtering/sorting/offset/limit operations on such a 
relation would be hard, not make sense, or be impossible.

Anyone more knowledgeable know if I''m wrong here?

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On Jul 20, 8:10 pm, Kendall Gifford
<zettab...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:
>
> I''m assuming this is because if #shapes *could* be/return a
relation then
> said relation would really have to have the ability to map to _n_ seperate
> SQL select statements (one for each table that can possibly be a
"child").
> As such, further filtering/sorting/offset/limit operations on such a
> relation would be hard, not make sense, or be impossible.
>
> Anyone more knowledgeable know if I''m wrong here?
You''ve pretty much hit the nail on the head - this issue is exactly
what the has_many_polymorphs plugin was created to handle. I can''t
find a definitive repo for a version that works on Rails 3, though.

--Matt Jones

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