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 ..

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?

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