Lookup fieldset using either string or symbol

active_model_serializers.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |spec|
   spec.homepage      = 'https://github.com/rails-api/active_model_serializers'
   spec.license       = 'MIT'
 
-  spec.files         = Dir["CHANGELOG.md", "MIT-LICENSE", "README.md", "lib/**/*"]
+  spec.files         = Dir['CHANGELOG.md', 'MIT-LICENSE', 'README.md', 'lib/**/*']
   spec.require_paths = ['lib']
   spec.executables   = []
 

lib/active_model/serializer/fieldset.rb

@@ -12,7 +12,7 @@ module ActiveModel
       end
 
       def fields_for(type)
-        fields[type.singularize.to_sym] || fields[type.pluralize.to_sym]
+        fields[type.to_s.singularize.to_sym] || fields[type.to_s.pluralize.to_sym]
       end
 
       protected

test/serializers/fieldset_test.rb

@@ -5,11 +5,21 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class FieldsetTest < ActiveSupport::TestCase
+      def setup
+        @fieldset = ActiveModel::Serializer::Fieldset.new('post' => %w(id title), 'comment' => ['body'])
+      end
+
       def test_fieldset_with_hash
-        fieldset = ActiveModel::Serializer::Fieldset.new('post' => %w(id title), 'comment' => ['body'])
         expected = { post: [:id, :title], comment: [:body] }
 
-        assert_equal(expected, fieldset.fields)
+        assert_equal(expected, @fieldset.fields)
+      end
+
+      def test_fields_for_accepts_string_or_symbol
+        expected = [:id, :title]
+
+        assert_equal(expected, @fieldset.fields_for(:post))
+        assert_equal(expected, @fieldset.fields_for('post'))
       end
     end
   end