When getting started with Ruby on Rails, associations between models can become quite confusing especially when there’s a thin line between two of them.

At first we’re tempted to use the one that makes more sense when thinking about it… For instance we can say that a page belongs to a book, but we could say that a page has one book too. The two of them establish a one-to-one association between both models.

But the question we need to ask ourselves here is: in which model do I want the foreign key to be? In fact, this is the slight difference between has_one and belongs_to in Ruby on Rails.

belongs_to will place the foreign key in the declaring model whereas has_one will place it in the other model.

Let’s see some examples, first using belongs_to.

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class Book < ActiveRecord::Base
end

class Page < ActiveRecord::Base
  belongs_to :book
end

This will use a book_id field in the pages table (note: of course you need to add that field with a migration). It also adds 4 methods in the Page class: book, book=, build_book, create_book.

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page = Page.create!
=> #<Page id: 1, book_id: nil>

book = Book.create!
=> #<Book id: 1>

page.book = book
=> #<Book id: 1>

page
=> #<Page id: 1, book_id: 1>

other_book = page.create_book!
=> #<Book id: 2>

page
=> #<Page id: 1, book_id: 2>

page.book
=> #<Book id: 2>

To make this a one-to-many association just declare the other side of it.

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class Book < ActiveRecord::Base
  has_many :pages
end

class Page < ActiveRecord::Base
  belongs_to :book
end

Now a book has many pages (and each page still belongs to a book) and you can use the usual methods on the book: pages, pages<<, pages.find, pages.build, pages.create and many more.

If we use a has_one association, here what happens:

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class Book < ActiveRecord::Base
end

class Page < ActiveRecord::Base
  has_one :book
end

Here are some examples:

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page = Page.create!
=> #<Page id: 1>

book = Book.create!
=> #<Book id: 1, page_id: nil>

page.book = book
=> #<Book id: 1, page_id: 1>

other_book = page.create_book!
=> #<Book id: 2, page_id: 1>

You will probably want to set the other side of the association at some point.

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class Book < ActiveRecord::Base
  belongs_to :page
end

class Page < ActiveRecord::Base
  has_one :book
end

But that way a book can only have one page…

To sum things up: use belongs_to when you want the foreign key in the declaring model, use has_one if you want it on the other model.

But anyway, you will rarely see a belongs_to or a has_one used alone. Most of the time it will be has_many with belongs_to for a one-to-many association and has_one with belongs_to for a one-to-one association.