[FIX] Fetch json key from item serializer if empty collection is passed to collection serializer and each_searializer is specified.

lib/active_model/serializer/collection_serializer.rb

@@ -10,8 +10,14 @@ module ActiveModel
       def initialize(resources, options = {})
         @root = options[:root]
         @object = resources
-        @serializers = resources.map do |resource|
+
         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_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)
+        serializers.first.try(:json_key).try(:pluralize)
-        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

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)

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