Rails has_one vs belongs_to semantics

Question

I have a model representing a Content item that contains some images. The number of images are fixed as these image references are very specific to the content.

Answer 1

The simple answer is to setup your associations in reverse of what you have, like so:

# app/models/content.rb
class Content < ActiveRecord::Base
  has_one :profile_image, :class_name => 'Image'
  has_one :background_image, :class_name => 'Image'
end

# app/models/image.rb
class Image < ActiveRecord::Base
  belongs_to :content
end

You don't need the foreign keys 'background_image_id' and 'profile_image_id' in the content table at all.

However, there's a more elegant solution: single table inheritance. Set it up now in case you want background and profile images to behave even slightly differently in the future, plus it will clarify your code today.

First, add a column to your images table called type:

# command line
script/generate migration AddTypeToImages type:string
rake db:migrate

Now setup your models like this:

# app/models/content.rb
class Content < ActiveRecord::Base
  has_one :profile_image
  has_one :background_image
end

# app/models/image.rb
class Image < ActiveRecord::Base
  belongs_to :content
end

# app/models/background_image.rb
class BackgroundImage < Image
  # background image specific code here
end

# app/models/profile_image.rb
class ProfileImage < Image
  # profile image specific code here
end

Now you can do all kinds of things like getting a list of all background images:

# script/console
BackgroundImage.all

This is more true to the data model you're trying to create, allows the easiest expandability in the future, and gives you some cool new methods today.

UPDATE:

I've since created a blog article called Single-Table Inheritance with Tests that goes into more detail, and covers testing.