[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
+
+        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