2 files changed, 22 insertions, 0 deletions
diff --git a/actionpack/lib/action_controller/metal/strong_parameters.rb b/actionpack/lib/action_controller/metal/strong_parameters.rb
index 8a2f63dfcd..11b7919b4b 100644
--- a/actionpack/lib/action_controller/metal/strong_parameters.rb
+++ b/actionpack/lib/action_controller/metal/strong_parameters.rb
@@ -94,6 +94,10 @@ module ActionController
       def each_element(object)
         if object.is_a?(Array)
           object.map { |el| yield el }.compact
+        elsif object.is_a?(Hash) && object.keys.all? { |k| k =~ /\A-?\d+\z/ }
+          hash = object.class.new
+          object.each { |k,v| hash[k] = yield v }
+          hash
         else
           yield object
         end
diff --git a/actionpack/test/controller/parameters/nested_parameters_test.rb b/actionpack/test/controller/parameters/nested_parameters_test.rb
index 16e8d2e528..ea67126cc5 100644
--- a/actionpack/test/controller/parameters/nested_parameters_test.rb
+++ b/actionpack/test/controller/parameters/nested_parameters_test.rb
@@ -92,4 +92,22 @@ class NestedParametersTest < ActiveSupport::TestCase
     permitted = params.permit book: { genre: :type }
     assert_nil permitted[:book][:genre]
   end
+
+  test "fields_for-style nested params" do
+    params = ActionController::Parameters.new({
+      book: {
+        authors_attributes: {
+          :'0' => { name: 'William Shakespeare', age_of_death: '52' },
+          :'-1' => { name: 'Unattributed Assistant' }
+        }
+      }
+    })
+    permitted = params.permit book: { authors_attributes: [ :name ] }
+
+    assert_not_nil permitted[:book][:authors_attributes]['0']
+    assert_not_nil permitted[:book][:authors_attributes]['-1']
+    assert_nil permitted[:book][:authors_attributes]['0'][:age_of_death]
+    assert_equal 'William Shakespeare', permitted[:book][:authors_attributes]['0'][:name]
+    assert_equal 'Unattributed Assistant', permitted[:book][:authors_attributes]['-1'][:name]
+  end
 end