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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