rails has_one of a has_many association

Question

I have a product, and a product can have many images. This is through an associations table. However, I would like a product to have one main image.

I know this is very easy to do with a method in the model, but I want it to be an association so that I can preload it in the query by using include.

Models:

class Product < ActiveRecord::Base
    has_many :image_associations, :as => :imageable
    has_many :images, :through => :image_associations
end

class ImageAssociation < ActiveRecord::Base
    belongs_to :image
    belongs_to :imageable, :polymorphic => true
end

class Image < ActiveRecord::Base
    has_many :image_associations
end

In the ImageAssociation table, there is a boolean column called 'feature' which associates an image as the 'main' image.

One of the ways I've thought about doing this is adding a main_image_id column to the products table, and then adding to the Image model:

belongs_to :image, :class => "Image", :foreign_key => "main_image_id"

However, this doesn't allow for any fallback to the other has_many images if the main image is nil -- which I'd like the association to load.

Which is why I was hoping for something in the images model like:

has_one :image, :through => :images, :conditions => 'feature = true', :order => 'created_at DESC'

But that gives me an association error.

Is there any way I can edit that has_one, or do I really need to run a rake task to push an image into every main_image_id field, and then add a validation for future products to make sure a main image has been added?

EDIT:

Should I be using a has_and_belongs_to_many association instead?

Answer 1

You're almost there, I think, although I'm not so clear on polymorphic associations. I think you want the following:

has_one :main_image_assoc, class => "ImageAssociation", :conditions => 'feature = true', :order => 'created_at DESC', :as => :imageable
has_one :image, :through => :main_image_assoc