From a813fa09a86ec522b43f16d137bd99e0a43985f5 Mon Sep 17 00:00:00 2001
From: Karol Galanciak <karol.galanciak@gmail.com>
Date: Sun, 5 Apr 2015 21:51:19 +0200
Subject: Accept lambda as child_index option in #fields_for method

---
 actionview/CHANGELOG.md                           |  4 ++++
 actionview/lib/action_view/helpers/form_helper.rb |  6 +++++-
 actionview/test/template/form_helper_test.rb      | 17 +++++++++++++++++
 3 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/actionview/CHANGELOG.md b/actionview/CHANGELOG.md
index 9d40bd6d5d..80aacf7234 100644
--- a/actionview/CHANGELOG.md
+++ b/actionview/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Accept lambda as `child_index` option in `fields_for` method.
+
+    *Karol Galanciak*
+
 *   `translate` allows `default: [[]]` again for a default value of `[]`.
 
     Fixes #19640.
diff --git a/actionview/lib/action_view/helpers/form_helper.rb b/actionview/lib/action_view/helpers/form_helper.rb
index 891cc53765..ece117b547 100644
--- a/actionview/lib/action_view/helpers/form_helper.rb
+++ b/actionview/lib/action_view/helpers/form_helper.rb
@@ -1928,7 +1928,11 @@ module ActionView
             explicit_child_index = options[:child_index]
             output = ActiveSupport::SafeBuffer.new
             association.each do |child|
-              options[:child_index] = nested_child_index(name) unless explicit_child_index
+              if explicit_child_index
+                options[:child_index] = explicit_child_index.call if explicit_child_index.respond_to?(:call)
+              else
+                options[:child_index] = nested_child_index(name)
+              end
               output << fields_for_nested_model("#{name}[#{options[:child_index]}]", child, options, block)
             end
             output
diff --git a/actionview/test/template/form_helper_test.rb b/actionview/test/template/form_helper_test.rb
index 4e336bea63..5c55b154d3 100644
--- a/actionview/test/template/form_helper_test.rb
+++ b/actionview/test/template/form_helper_test.rb
@@ -2878,6 +2878,23 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_with_child_index_as_lambda_option_override_on_a_nested_attributes_collection_association
+    @post.comments = []
+
+    form_for(@post) do |f|
+      concat f.fields_for(:comments, Comment.new(321), child_index: -> { 'abc' } ) { |cf|
+        concat cf.text_field(:name)
+      }
+    end
+
+    expected = whole_form('/posts/123', 'edit_post_123', 'edit_post', method: 'patch') do
+      '<input id="post_comments_attributes_abc_name" name="post[comments_attributes][abc][name]" type="text" value="comment #321" />' +
+      '<input id="post_comments_attributes_abc_id" name="post[comments_attributes][abc][id]" type="hidden" value="321" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   class FakeAssociationProxy
     def to_ary
       [1, 2, 3]
-- 
cgit v1.2.3