Fixing select[multiple] html specification problem.

Generating hidden input with same name before each multiple select

actionpack/lib/action_view/helpers/form_options_helper.rb

@@ -128,6 +128,28 @@ module FormOptionsHelper
       # By default, <tt>post.person_id</tt> is the selected option. Specify <tt>:selected => value</tt> to use a different selection
       # or <tt>:selected => nil</tt> to leave all options unselected. Similarly, you can specify values to be disabled in the option
       # tags by specifying the <tt>:disabled</tt> option. This can either be a single value or an array of values to be disabled.
+      #
+      # ==== Gotcha
+      #
+      # The HTML specification says when +multiple+ parameter passed to select and all options got deselected 
+      # web browsers do not send any value to server. Unfortunately this introduces a gotcha:
+      # if an +User+ model has many +roles+ and have +role_ids+ accessor, and in the form that edits roles of the user
+      # the user deselects all roles from +role_ids+ multiple select box, no +role_ids+ parameter is sent. So,
+      # any mass-assignment idiom like
+      #
+      #   @user.update_attributes(params[:user])
+      #
+      # wouldn't update roles.
+      #
+      # To prevent this the helper generates an auxiliary hidden field before
+      # every multiple select. The hidden field has the same name as multiple select and blank value.
+      #
+      # This way, the client either sends only the hidden field (representing
+      # the deselected multiple select box), or both fields. Since the HTML specification
+      # says key/value pairs have to be sent in the same order they appear in the
+      # form, and parameters extraction gets the last occurrence of any repeated
+      # key in the query string, that works for ordinary forms.
+      #
       def select(object, method, choices, options = {}, html_options = {})
         InstanceTag.new(object, method, self, options.delete(:object)).to_select_tag(choices, options, html_options)
       end
@@ -557,7 +579,7 @@ def to_select_tag(choices, options, html_options)
         value = value(object)
         selected_value = options.has_key?(:selected) ? options[:selected] : value
         disabled_value = options.has_key?(:disabled) ? options[:disabled] : nil
-        content_tag("select", add_options(options_for_select(choices, :selected => selected_value, :disabled => disabled_value), options, selected_value), html_options)
+        select_content_tag(add_options(options_for_select(choices, :selected => selected_value, :disabled => disabled_value), options, selected_value), html_options)
       end
 
       def to_collection_select_tag(collection, value_method, text_method, options, html_options)
@@ -566,8 +588,8 @@ def to_collection_select_tag(collection, value_method, text_method, options, htm
         value = value(object)
         disabled_value = options.has_key?(:disabled) ? options[:disabled] : nil
         selected_value = options.has_key?(:selected) ? options[:selected] : value
-        content_tag(
-          "select", add_options(options_from_collection_for_select(collection, value_method, text_method, :selected => selected_value, :disabled => disabled_value), options, value), html_options
+        select_content_tag(
+          add_options(options_from_collection_for_select(collection, value_method, text_method, :selected => selected_value, :disabled => disabled_value), options, value), html_options
         )
       end
 
@@ -575,8 +597,8 @@ def to_grouped_collection_select_tag(collection, group_method, group_label_metho
         html_options = html_options.stringify_keys
         add_default_name_and_id(html_options)
         value = value(object)
-        content_tag(
-          "select", add_options(option_groups_from_collection_for_select(collection, group_method, group_label_method, option_key_method, option_value_method, value), options, value), html_options
+        select_content_tag(
+          add_options(option_groups_from_collection_for_select(collection, group_method, group_label_method, option_key_method, option_value_method, value), options, value), html_options
         )
       end
 
@@ -584,7 +606,7 @@ def to_time_zone_select_tag(priority_zones, options, html_options)
         html_options = html_options.stringify_keys
         add_default_name_and_id(html_options)
         value = value(object)
-        content_tag("select",
+        select_content_tag(
           add_options(
             time_zone_options_for_select(value || options[:default], priority_zones, options[:model] || ActiveSupport::TimeZone),
             options, value
@@ -603,6 +625,15 @@ def add_options(option_tags, options, value = nil)
           end
           option_tags.html_safe
         end
+
+        def select_content_tag(content, html_options)
+          select = content_tag("select", content, html_options)
+          if html_options["multiple"]
+            tag("input", :disabled => html_options["disabled"], :name => html_options["name"], :type => "hidden", :value => "") + select 
+          else
+            select
+          end
+        end
     end
 
     class FormBuilder

actionpack/test/template/form_options_helper_test.rb

@@ -457,6 +457,22 @@ def test_select_under_fields_for_with_string_and_given_prompt
     )
   end
 
+  def test_select_with_multiple_to_add_hidden_input
+    output_buffer =  select(:post, :category, "", {}, :multiple => true)
+    assert_dom_equal(
+      "<input type=\"hidden\" name=\"post[category][]\" value=\"\"/><select multiple=\"multiple\" id=\"post_category\" name=\"post[category][]\"></select>",
+      output_buffer
+    )
+  end
+
+  def test_select_with_multiple_and_disabled_to_add_disabled_hidden_input
+    output_buffer =  select(:post, :category, "", {}, :multiple => true, :disabled => true)
+    assert_dom_equal(
+      "<input disabled=\"disabled\"type=\"hidden\" name=\"post[category][]\" value=\"\"/><select multiple=\"multiple\" disabled=\"disabled\" id=\"post_category\" name=\"post[category][]\"></select>",
+      output_buffer
+    )
+  end
+
   def test_select_with_blank
     @post = Post.new
     @post.category = "<mus>"
@@ -649,11 +665,11 @@ def test_collection_select_with_blank_as_string_and_style
     )
   end
 
-  def test_collection_select_with_multiple_option_appends_array_brackets
+  def test_collection_select_with_multiple_option_appends_array_brackets_and_hidden_input
     @post = Post.new
     @post.author_name = "Babe"
 
-    expected = "<select id=\"post_author_name\" name=\"post[author_name][]\" multiple=\"multiple\"><option value=\"\"></option>\n<option value=\"&lt;Abe&gt;\">&lt;Abe&gt;</option>\n<option value=\"Babe\" selected=\"selected\">Babe</option>\n<option value=\"Cabe\">Cabe</option></select>"
+    expected = "<input type=\"hidden\" name=\"post[author_name][]\" value=\"\"/><select id=\"post_author_name\" name=\"post[author_name][]\" multiple=\"multiple\"><option value=\"\"></option>\n<option value=\"&lt;Abe&gt;\">&lt;Abe&gt;</option>\n<option value=\"Babe\" selected=\"selected\">Babe</option>\n<option value=\"Cabe\">Cabe</option></select>"
 
     # Should suffix default name with [].
     assert_dom_equal expected, collection_select("post", "author_name", dummy_posts, "author_name", "author_name", { :include_blank => true }, :multiple => true)