Rails5 > RSpec > ActiveModel::Serializers Raw

active_model_serializers.rb

# spec/support/active_model_serializers.rb

# inspired by: https://sonjapeterson.github.io/2016/03/19/testing-serializers.html
module SerializerSpecHelper
  def serialize(obj, opts = {})
    serializer_class = opts.delete(:serializer_class) || "#{obj.class.name}Serializer".constantize
    serializer = serializer_class.send(:new, obj)
    serializer.serializable_hash.with_indifferent_access
  end
end

RSpec.configure do |config|
  # ActiveModel Serializer Helper
  config.include SerializerSpecHelper, type: :serializer

  # set `:type` for serializers directory
  config.define_derived_metadata(file_path: Regexp.new("/spec/serializers")) do |metadata|
    metadata[:type] = :serializer
  end
end

user_serializer.rb

# app/serializers/user_serializer.rb
class UserSerializer < ActiveModel::Serializer
  attributes :id, :name
end

user_serializer_spec.rb

# spec/serializers/user_serializer_spec.rb
require "rails_helper"

RSpec.describe UserSerializer do
  let(:user) { create :user }
  subject { serialize(user) }

  it { is_expected.to include(id: user.id, name: user.name) }
end

you forgot to include type: :serializer in your spec