Rails: Up and Running, 2nd Edition

has_one

The simplest database relationship is the one-to-one relationship. With Active Record, you can implement one-to-one relationships with belongs_to or has_one. You’ll decide which to use based on where the foreign key lands. For example, if an Address belonged to a Person, the addresses table would need a person_id column. Person would use a has_one relationship. Figure 4-3 shows a has_one relationship.

Figure 4-3. In this one-to-one relationship, a Photo has_one File

Let’s take a simple example. Hypothetically, you could have decided to implement photos and files in separate tables. If you put a foreign key called photo_id into the files table, you would have this Active Record Photo class:

class Photo < ActiveRecord::Base
  has_one :file
  ...
end

has_one is identical to belongs_to with respect to metaprogramming. For example, adding has_one :photo or belongs_to :photo to Slide would add the photo attribute to Slide. But the Photo Share application does not need a separate table to manage files, so let’s move on to that marathon of typing, has_and_belongs_to_many.