From 2dd0c334616e208116f8dd5d047d9d23c8b1bc83 Mon Sep 17 00:00:00 2001
From: Roman Kapitonov <RomanKapitonov@coherentsolutions.com>
Date: Wed, 24 Feb 2016 21:03:36 +0300
Subject: [PATCH] [FIX] Fetch json key from item serializer if empty collection
 is passed to collection serializer and each_searializer is specified.

---
 .../serializer/collection_serializer.rb       | 21 +++++++++++++++----
 test/collection_serializer_test.rb            |  6 ++++++
 test/fixtures/poro.rb                         |  6 ++++++
 3 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/lib/active_model/serializer/collection_serializer.rb b/lib/active_model/serializer/collection_serializer.rb
index 7862e994..ebaded3d 100644
--- a/lib/active_model/serializer/collection_serializer.rb
+++ b/lib/active_model/serializer/collection_serializer.rb
@@ -10,8 +10,14 @@ module ActiveModel
       def initialize(resources, options = {})
         @root = options[:root]
         @object = resources
+
+        serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
+
+        if resources.blank? && options[:serializer]
+          @each_serializer = options[:serializer]
+        end
+
         @serializers = resources.map do |resource|
-          serializer_context_class = options.fetch(:serializer_context_class, ActiveModel::Serializer)
           serializer_class = options.fetch(:serializer) { serializer_context_class.serializer_for(resource) }
 
           if serializer_class.nil? # rubocop:disable Style/GuardClause
@@ -27,7 +33,7 @@ module ActiveModel
       end
 
       def json_key
-        root || derived_root
+        root || derived_root || guess_root || default_root
       end
 
       def paginated?
@@ -43,8 +49,15 @@ module ActiveModel
       private
 
       def derived_root
-        key = serializers.first.try(:json_key) || object.try(:name).try(:underscore)
-        key.try(:pluralize)
+        serializers.first.try(:json_key).try(:pluralize)
+      end
+
+      def default_root
+        object.try(:name).try(:underscore).try(:pluralize)
+      end
+
+      def guess_root
+        @each_serializer.try(:allocate).try(:json_key).try(:pluralize)
       end
     end
   end
diff --git a/test/collection_serializer_test.rb b/test/collection_serializer_test.rb
index a7c0fa02..833867b0 100644
--- a/test/collection_serializer_test.rb
+++ b/test/collection_serializer_test.rb
@@ -84,6 +84,12 @@ module ActiveModel
         assert_nil serializer.json_key
       end
 
+      def test_json_key_with_empty_resources_with_serializer
+        resource = []
+        serializer = collection_serializer.new(resource, serializer: MessagesSerializer)
+        assert_equal 'messages', serializer.json_key
+      end
+
       def test_json_key_with_root
         expected = 'custom_root'
         serializer = collection_serializer.new(@resource, root: expected)
diff --git a/test/fixtures/poro.rb b/test/fixtures/poro.rb
index c7fb831c..7fc59ffa 100644
--- a/test/fixtures/poro.rb
+++ b/test/fixtures/poro.rb
@@ -183,6 +183,12 @@ PaginatedSerializer = Class.new(ActiveModel::Serializer::CollectionSerializer) d
   end
 end
 
+MessagesSerializer = Class.new(ActiveModel::Serializer) do
+  def json_key
+    'messages'
+  end
+end
+
 AlternateBlogSerializer = Class.new(ActiveModel::Serializer) do
   attribute :id
   attribute :name, key: :title