render json: {'user': {name: 'Peter' }}

def index
  render json: @users, serializer: UserSerializer, adapter: :json_api
end

class UsersController < ApplicationController
  before_action :set_user, only: [:show, :update, :destroy]
  ...
  ...
  # GET /users/1
  def show
    render json: @user
  end
  ...
  ...
  private
   # Use callbacks to share common setup or constraints between actions.
   def set_user
     @user = User.find(params[:id])
   end

   def user_params
     ActiveModelSerializers::Deserialization.jsonapi_parse(params)
   end
end

class UserSerializer < ActiveModel::Serializer
  attributes :id, :name, :email, :outro_atributo

  belongs_to :alguma_associação
  has_many :outra_associação
  has_one :alguma_outra_associação
end