Failed to serialize the response in Web API with Json

If you are working with EF, besides adding the code below on Global.asax

GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings
    .ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;
GlobalConfiguration.Configuration.Formatters
    .Remove(GlobalConfiguration.Configuration.Formatters.XmlFormatter);          

Dont`t forget to import

using System.Data.Entity;

Then you can return your own EF Models

Simple as that!