Merge pull request #17973 from maurogeorge/file_field_hidden_field

Generate a hidden_tag when using a file_field

actionview/CHANGELOG.md

@@ -1,6 +1,12 @@
+*   Add a `hidden_field` on the `file_field` to avoid raise a error when the only
+    input on the form is the `file_field`.
+
+    *Mauro George*
+
 *   Add an explicit error message, in `ActionView::PartialRenderer` for partial
     `rendering`, when the value of option `as` has invalid characters.
 
     *Angelo Capilleri*
 
+
 Please check [4-2-stable](https://github.com/rails/rails/blob/4-2-stable/actionview/CHANGELOG.md) for previous changes.

actionview/lib/action_view/helpers/form_helper.rb

@@ -854,6 +854,23 @@ def hidden_field(object_name, method, options = {})
       #
       #   file_field(:attachment, :file, class: 'file_input')
       #   # => <input type="file" id="attachment_file" name="attachment[file]" class="file_input" />
+      #
+      # ==== Gotcha
+      #
+      # The HTML specification says when nothing is select on a file field web browsers do not send any value to server.
+      # Unfortunately this introduces a gotcha:
+      # if an +User+ model has a +avatar+ field, and in the form none file is selected no +avatar+ parameter is sent. So,
+      # any mass-assignment idiom like
+      #
+      #   @user.update(params[:user])
+      #
+      # wouldn't update avatar.
+      #
+      # To prevent this the helper generates an auxiliary hidden field before
+      # every file field. The hidden field has the same name as file field and blank value.
+      #
+      # In case if you don't want the helper to generate this hidden field you can specify
+      # <tt>include_hidden: false</tt> option.
       def file_field(object_name, method, options = {})
         Tags::FileField.new(object_name, method, self, options).render
       end

actionview/lib/action_view/helpers/tags/file_field.rb

@@ -2,6 +2,21 @@ module ActionView
   module Helpers
     module Tags # :nodoc:
       class FileField < TextField # :nodoc:
+
+        def render
+          options = @options.stringify_keys
+
+          if options.fetch("include_hidden", true)
+            add_default_name_and_id(options)
+            options[:type] = "file"
+            tag("input", name: options["name"], type: "hidden", value: "") + tag("input", options)
+          else
+            options.delete("include_hidden")
+            @options = options
+
+            super
+          end
+        end
       end
     end
   end

actionview/test/template/form_helper_test.rb

@@ -472,18 +472,33 @@ def test_text_field_doesnt_change_param_values
     assert_dom_equal expected, text_field(object_name, "title")
   end
 
-  def test_file_field_has_no_size
+  def test_file_field_does_generate_a_hidden_field
+    expected = '<input name="user[avatar]" type="hidden" value="" /><input id="user_avatar" name="user[avatar]" type="file" />'
+    assert_dom_equal expected, file_field("user", "avatar")
+  end
+
+  def test_file_field_does_not_generate_a_hidden_field_if_included_hidden_option_is_false
     expected = '<input id="user_avatar" name="user[avatar]" type="file" />'
+    assert_dom_equal expected, file_field("user", "avatar", include_hidden: false)
+  end
+
+  def test_file_field_does_not_generate_a_hidden_field_if_included_hidden_option_is_false_with_key_as_string
+    expected = '<input id="user_avatar" name="user[avatar]" type="file" />'
+    assert_dom_equal expected, file_field("user", "avatar", "include_hidden" => false)
+  end
+
+  def test_file_field_has_no_size
+    expected = '<input name="user[avatar]" type="hidden" value="" /><input id="user_avatar" name="user[avatar]" type="file" />'
     assert_dom_equal expected, file_field("user", "avatar")
   end
 
   def test_file_field_with_multiple_behavior
-    expected = '<input id="import_file" multiple="multiple" name="import[file][]" type="file" />'
+    expected = '<input name="import[file][]" type="hidden" value="" /><input id="import_file" multiple="multiple" name="import[file][]" type="file" />'
     assert_dom_equal expected, file_field("import", "file", :multiple => true)
   end
 
   def test_file_field_with_multiple_behavior_and_explicit_name
-    expected = '<input id="import_file" multiple="multiple" name="custom" type="file" />'
+    expected = '<input name="custom" type="hidden" value="" /><input id="import_file" multiple="multiple" name="custom" type="file" />'
     assert_dom_equal expected, file_field("import", "file", :multiple => true, :name => "custom")
   end
 
@@ -1719,7 +1734,7 @@ def test_form_for_with_file_field_generate_multipart
     end
 
     expected = whole_form("/posts/123", "create-post", "edit_post", method: "patch", multipart: true) do
-      "<input name='post[file]' type='file' id='post_file' />"
+      "<input name='post[file]' type='hidden' value='' /><input name='post[file]' type='file' id='post_file' />"
     end
 
     assert_dom_equal expected, output_buffer
@@ -1735,7 +1750,7 @@ def test_fields_for_with_file_field_generate_multipart
     end
 
     expected = whole_form("/posts/123", "edit_post_123", "edit_post", method: "patch", multipart: true) do
-      "<input name='post[comment][file]' type='file' id='post_comment_file' />"
+      "<input name='post[comment][file]' type='hidden' value='' /><input name='post[comment][file]' type='file' id='post_comment_file' />"
     end
 
     assert_dom_equal expected, output_buffer