Edit the migrations:

# db/migrations/20171024081558_create_authors.rb
Hanami::Model.migration do
  change do
    create_table :authors do
      primary_key :id
      column :name,       String,   null: false
      column :created_at, DateTime, null: false
      column :updated_at, DateTime, null: false
    end
  end
end

# db/migrations/20171024081617_create_books.rb
Hanami::Model.migration do
    create_table :books do
      primary_key :id
      foreign_key :author_id, :authors, on_delete: :cascade, null: false
      column :title,      String,   null: false
      column :created_at, DateTime, null: false
      column :updated_at, DateTime, null: false
    end
  end
end

Now we can prepare the database:

Let’s edit BookRepository with the following code:

# lib/bookshelf/repositories/book_repository.rb
class BookRepository < Hanami::Repository
  associations do
    belongs_to :author
  end
  def find_with_author(id)
    aggregate(:author).where(id: id).map_to(Book).one
  end

Let’s create a book:

repository = BookRepository.new
book = repository.create(author_id: 1, title: "Hanami")
  # => #<Book:0x00007f89ac270118 @attributes={:id=>1, :author_id=>1, :created_at=>2017-10-24 08:25:41 UTC, :updated_at=>2017-10-24 08:25:41 UTC}>

What happens if we load the author with BookRepository#find?

Because we haven’t the associated records, book.author is nil. We can use the method that we have defined on before (#find_with_author):

book = repository.find_with_author(book.id)
  # => #<Book:0x00007fb3f88896a0 @attributes={:id=>1, :author_id=>1, :created_at=>2017-10-24 08:25:41 UTC, :updated_at=>2017-10-24 08:25:41 UTC, :author=>#<Author:0x00007fb3f8888980 @attributes={:id=>1, :name=>"Luca", :created_at=>2017-10-24 08:25:15 UTC, :updated_at=>2017-10-24 08:25:15 UTC}>}>
book.author
  # => => #<Author:0x00007fb3f8888980 @attributes={:id=>1, :name=>"Luca", :created_at=>2017-10-24 08:25:15 UTC, :updated_at=>2017-10-24 08:25:15 UTC}>

What if we need to unassociate a book from its author?

Because we declared a foreign key with the , we cannot set author_id to NULL or to reference to an unexisting author. This is a mechanism against orphaned records: a book is forced to reference a valid author.

The only way to remove a book from an author is to delete the book record.

repository.delete(book.id)
  # => nil