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
  change do
    create_table :books do
      primary_key :id
      foreign_key :author_id, :authors, on_delete: :cascade
      column :title,      String,   null: false
      column :created_at, DateTime, null: false
      column :updated_at, DateTime, null: false
    end
end

Now we can prepare the database:

$ bundle exec hanami db prepare

Let’s edit AuthorRepository with the following code:

Let’s create an author with a collection of books with a single database operation:

repository = AuthorRepository.new
author = repository.create_with_books(name: "Alexandre Dumas", books: [{title: "The Count of Montecristo"}])
  # => #<Author:0x007f811c415420 @attributes={:id=>1, :name=>"Alexandre Dumas", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC, :books=>[#<Book:0x007f811c40fe08 @attributes={:id=>1, :author_id=>1, :title=>"The Count of Montecristo", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC}>]}>
author.id
  # => 1
author.name
  # => "Alexandre Dumas"
author.books
  # => [#<Book:0x007f811c40fe08 @attributes={:id=>1, :author_id=>1, :title=>"The Count of Montecristo", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC}>]

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

author = repository.find(author.id)
  # => #<Author:0x007f811b6237e0 @attributes={:id=>1, :name=>"Alexandre Dumas", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC}>
author.books
  # => nil

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

author = repository.find_with_books(author.id)
  # => #<Author:0x007f811bbeb6f0 @attributes={:id=>1, :name=>"Alexandre Dumas", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC, :books=>[#<Book:0x007f811bbea430 @attributes={:id=>1, :author_id=>1, :title=>"The Count of Montecristo", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC}>]}>
  # => [#<Book:0x007f811bbea430 @attributes={:id=>1, :author_id=>1, :title=>"The Count of Montecristo", :created_at=>2016-11-15 09:19:38 UTC, :updated_at=>2016-11-15 09:19:38 UTC}>]

What if we need to add or remove books from an author? We need to define new methods to do so.

Let’s add a book:

book = repository.add_book(author, title: "The Three Musketeers")

And remove it:

repository.remove_book(author, book.id)

# lib/bookshelf/repositories/author_repository.rb
class AuthorRepository < Hanami::Repository
  # ...
  def books_count(author)
    assoc(:books, author).count
  end
  def on_sales_books_count(author)
    assoc(:books, author).where(on_sale: true).count
  end
  def book_exists?(author, id)
    book_for(author, id).exists?
  end
  private
  def book_for(author, id)
    assoc(:books, author).where(id: id)
  end

You can also run operations on top of these scopes: