I'm in Rails. How to set up a model so each element of Collage array “has_one” Work?

has_one is for one-to-one relationships. That isn't what you want in this case.

If you were using mongoid, this is one design that could work for you. In it, there is a Collage class which embeds many slides.

class Collage
  include Mongoid::Document
  embeds_many :slides
end

class Slide
  include Mongoid::Document
  embedded_in :collage
  belongs_to :works
end

class Work
  include Mongoid::Document
  has_many :Slides
  field :image_url, type: String
end

If you don't need the helper methods on the classes just create arrays and embedded documents yourself and manipulate them using the Ruby driver as needed. Essentially these class definitions would create the following collage documents:

Collage:

{
  "slides": [
    {
      "works": [ work_id, work_id , ....]
    },
    {
      "works": [ work_id, work_id , ....]
    }
  ]
}