gem 'grape'
gem 'grape_fast_jsonapi'

class API < Grape::API
  content_type :jsonapi, "application/vnd.api+json"
  formatter :json, Grape::Formatter::FastJsonapi
  formatter :jsonapi, Grape::Formatter::FastJsonapi
end

get "/" do
  user = User.find("123")
  render user, include: [:account]
end

get "/" do
  user = User.find("123")
  render user, serializer: 'CustomUserSerializer'
end

get "/" do
  user = User.find("123")
  render CustomUserSerializer.new(user).serialized_json
end

# FastJsonapi serializer example
# app/serializers/user_serializer.rb
class UserSerializer
  include FastJsonapi::ObjectSerializer

  set_type :user
  has_many :orders

  attributes :name, :email
end

# config/initializers/grape_swagger.rb
GrapeSwagger.model_parsers.register(GrapeSwagger::FastJsonapi::Parser, UserSerializer)

# Your grape API endpoint
desc 'Get current user',
  success: { code: 200, model: UserSerializer, message: 'The current user' }
# [...]