Add #one to HasMany (hanami#550)

Per hanami#550, the HasMany interface
does not line up with Hanami's guides. Prior to this commit, the
following is invalid as HasMany does not implement #one.

```ruby
class AuthorRepository < Hanami::Repository
  associations do
    has_many :books
  end

  def find_book(author, id)
    book_for(author, id).one
  end

  private

  def book_for(author, id)
    assoc(:books, author).where(id: id)
  end
end
```

This implementation is based on a couple of others, notably ROM's and
Ecto's (Elixir). I opted to name the raised error MultipleResultsError,
which is the exact name used by Ecto, because I feel it bears a semantic
relationship to the name of the method from which raises it.

lib/hanami/model/associations/has_many.rb

@@ -46,6 +46,16 @@ def initialize(repository, source, target, subject, scope = nil)
           freeze
         end
 
+        # @raise [MultipleResultsError] if more than one record is available
+        #
+        # @since 1.3.3
+        # @api private
+        def one
+          raise Hanami::Model::MultipleResultsError, "#{count} results returned" if count > 1
+
+          scope.one
+        end
+
         # @since 0.7.0
         # @api private
         def create(data)

lib/hanami/model/error.rb

@@ -127,5 +127,8 @@ def initialize(url)
         super("Unknown database adapter for URL: #{url.inspect}. Please check your database configuration (hint: ENV['DATABASE_URL']).")
       end
     end
+
+    class MultipleResultsError < Error
+    end
   end
 end

spec/integration/hanami/model/associations/has_many_spec.rb

@@ -192,4 +192,33 @@
     # skipped spec
     it '#remove'
   end
+
+  describe '#one' do
+    it 'raises an error if more than one record exists' do
+      author = authors.create(name: 'Umberto Eco')
+      book = books.create(author_id: author.id, title: 'Foucault Pendulum')
+      book = books.create(author_id: author.id, title: 'Foucault Pendulum')
+
+      expect do
+        authors.find_book_by_title(author, book.title)
+      end.to raise_error(Hanami::Model::MultipleResultsError)
+    end
+
+    it 'returns an individual record if only one record exists' do
+      author = authors.create(name: 'Umberto Eco')
+      book = books.create(author_id: author.id, title: 'Foucault Pendulum')
+
+      found = authors.find_book(author, book.id)
+
+      expect(found).to eq(book)
+    end
+
+    it 'returns nil if no records exist' do
+      author = authors.create(name: 'Aspiring Author')
+
+      found = authors.find_book(author, nil)
+
+      expect(found).to eq(nil)
+    end
+  end
 end

spec/support/fixtures.rb

@@ -258,12 +258,20 @@ def find_book(author, id)
     book_for(author, id).one
   end
 
+  def find_book_by_title(author, title)
+    book_by_title(author, title).one
+  end
+
   def book_exists?(author, id)
     book_for(author, id).exists?
   end
 
   private
 
+  def book_by_title(author, title)
+    assoc(:books, author).where(title: title)
+  end
+
   def book_for(author, id)
     assoc(:books, author).where(id: id)
   end