Ensures there is a root available for arrays

lib/grape-active_model_serializers/formatter.rb

@@ -13,6 +13,11 @@ module Grape
           @endpoint = env["api.endpoint"]
           options   = endpoint.namespace_options.merge(endpoint.route_options)
 
+          if resource.is_a?(Array) && !resource.empty?
+            # ensure we have an root to fallback on
+            endpoint.controller_name = resource.first.class.name.underscore.pluralize
+          end
+
           serializer = ::ActiveModel::Serializer.build_json(endpoint, resource, options)
 
           if serializer

lib/grape/endpoint_extension.rb

@@ -6,6 +6,8 @@
 #
 module Grape
   module EndpointExtension
+    attr_accessor :controller_name
+
     def namespace_options
       settings[:namespace] ? settings[:namespace].options : {}
     end
@@ -18,7 +20,6 @@ module Grape
     def serialization_scope; end
     def _serialization_scope; end
     def url_options; end
-    def controller_name; end
   end
 
   Endpoint.send(:include, EndpointExtension)