add multiple option possibility to select

lib/hanami/helpers/form_helper/form_builder.rb

@@ -780,17 +780,30 @@ def password_field(name, attributes = {})
         #   #    <option value="it" selected="selected">Italy</option>
         #   #    <option value="us">United States</option>
         #   #  </select>
+        #
+        # @example Multiple select
+        #   <%=
+        #     # ...
+        #     values = Hash['it' => 'Italy', 'us' => 'United States']
+        #     select :stores, values, multiple: true
+        #   %>
+        #
+        #   # Output:
+        #   # <select name="book[store][]" id="book-store" multiple="multiple">
+        #   #   <option value="it">Italy</option>
+        #   #    <option value="us">United States</option>
+        #   #  </select>
         def select(name, values, attributes = {}) # rubocop:disable Metrics/AbcSize, Metrics/MethodLength
           options    = attributes.delete(:options) { {} }
-          attributes = { name: _input_name(name), id: _input_id(name) }.merge(attributes)
+          attributes = { name: _select_input_name(name, attributes[:multiple]), id: _input_id(name) }.merge(attributes)
           prompt     = options.delete(:prompt)
           selected   = options.delete(:selected)
 
           super(attributes) do
             option(prompt) unless prompt.nil?
 
             values.each do |content, value|
-              if selected == value || _value(name) == value
+              if _select_option_selected?(value, selected, _value(name), attributes[:multiple])
                 option(content, { value: value, selected: SELECTED }.merge(options))
               else
                 option(content, { value: value }.merge(options))
@@ -1030,6 +1043,17 @@ def _attributes_for_check_box(name, attributes)
 
           attributes
         end
+
+        def _select_input_name(name, multiple)
+          select_name = _input_name(name)
+          select_name = "#{select_name}[]" if multiple
+          select_name
+        end
+
+        def _select_option_selected?(value, selected, input_value, multiple)
+          value == selected || (multiple && (selected.is_a?(Array) && selected.include?(value))) ||
+            value == input_value || (multiple && (input_value.is_a?(Array) && input_value.include?(value)))
+        end
       end
     end
   end

test/form_helper_test.rb

@@ -1175,6 +1175,24 @@
       actual.must_include %(<select name="book[store]" id="book-store">\n<option value="it" class="form-option">Italy</option>\n<option value="us" class="form-option">United States</option>\n</select>)
     end
 
+    describe "with option 'multiple'" do
+      it 'renders' do
+        actual = view.form_for(:book, action) do
+          select :store, values, multiple: true
+        end.to_s
+
+        actual.must_include %(<select name="book[store][]" id="book-store" multiple="multiple">\n<option value="it">Italy</option>\n<option value="us">United States</option>\n</select>)
+      end
+
+      it 'allows to select values' do
+        actual = view.form_for(:book, action) do
+          select :store, values, multiple: true, options: { selected: %w(it us) }
+        end.to_s
+
+        actual.must_include %(<select name="book[store][]" id="book-store" multiple="multiple">\n<option value="it" selected="selected">Italy</option>\n<option value="us" selected="selected">United States</option>\n</select>)
+      end
+    end
+
     describe 'with values an structured Array of values' do
       let(:values) { [%w(Italy it), ['United States', 'us']] }