Question

I know this topic has been approached several times before in various ways, but I haven’t been able to synthesize the other questions to meet this particular use case for Rails 3:

I am trying to construct a data model in which there are Attachments and Attachables. Any Attachment can be connected with an Attachable, and vice-versa. I would like this to ultimately be used as a combination acts_as plugin (acts_as_attachable and acts_as_attachment).

My current schema uses an Attacher model with two polymorphic belongs_to relationships to attempt to accomplish this.

The specific case outlined here involves an Attachable model (Good) and two Attachment models (Image and Movie).

Here is what I have so far:

attacher.rb

class Attacher < ActiveRecord::Base
    belongs_to :attachment, :polymorphic => true
    belongs_to :attachable, :polymorphic => true
end

good.rb

class Good < ActiveRecord::Base
    has_many :attachment_relations, :as => :attachable, :class_name => 'Attacher'
    has_many :attached_images, :through => :attachment_relations, :source => :attachment, :source_type  => 'Image'
    has_many :attached_movies, :through => :attachment_relations, :source => :attachment, :source_type  => 'Movie'
end

image.rb

class Image < ActiveRecord::Base
    has_many :attachable_relations, :as => :attachment, :class_name => 'Attacher'
end

movie.rb

class Movie < ActiveRecord::Base
    has_many :attachable_relations, :as => :attachment, :class_name => 'Attacher'
end

This works insofar as I can do the following:

@g = Good.create!
@i = Image.create!
@m = Movie.create!

@g.attached_images << @i
@g.attached_images       # [#<Image id: 1 ... >]

@g.attached_movies << @m
@g.attached_movies       # [#<Movie id: 1 ... >]

@g.attachment_relations  # [#<Attacher id: 1, attachable_id: 1, attachable_type: "Good", attachment_id: 1, attachment_type: "Image" ...>, #<Attacher id: 2, attachable_id: 1, attachable_type: "Good", attachment_id: 1, attachment_type: "Movie" ...>]

The thrust of my question is: can I use associations to construct the following method/return so I can still do things like @g.attachments.where( ... ):

@g.attachments           # [#<Image id: 1 ... >, #<Movie id: 1 ... >]

I have tried a bunch of things like this:

good.rb

class Good < ActiveRecord::Base
    has_many :attachment_relations, :as => :attachable, :class_name => 'Attacher'
    has_many :attachments, :through => :attachment_relations
end

But they tend to throw me this:

ActiveRecord::HasManyThroughAssociationPolymorphicError: Cannot have a has_many :through association 'Good#attachments' on the polymorphic object 'Attachment#attachment'

I think I basically need something that could act like source_type => :any …

Also, this is my first question on StackOverflow, so please let me know if there is anything I can do to improve my question. Thanks in advance! 🙂

Answer 1

Unfortunately, I have come to the conclusion that there isn’t an easy or standard way to do this. The clearest basic solution I have found is from Squeegy’s answer to this question: Rails polymorphic many to many association, where he references his gist to a reasonably simple has_relations module.

I think this basically answers my question and serves a good jumping-off point for creating a more robust solution.