NewtonSoftJsonResult.cs

public class NewtonSoftJsonResult : JsonResult
{
    public override void ExecuteResult(ControllerContext context)
    {
        if (context == null)
            throw new ArgumentNullException("context");

        var response = context.HttpContext.Response;

        response.ContentType = ContentType;
        response.ContentEncoding = ContentEncoding;

        if (Data == null)
            return;

        var serializer = new JsonSerializerSettings
        {
            ReferenceLoopHandling = ReferenceLoopHandling.Ignore,
            ContractResolver = new NHibernateContractResolver()
        };
        string serializedObject = JsonConvert.SerializeObject(Data, serializer);

        response.Write(serializedObject);
    }
}

public class NHibernateContractResolver : DefaultContractResolver
{
    protected override JsonContract CreateContract(Type objectType)
    {
        if (typeof(NHibernate.Proxy.INHibernateProxy).IsAssignableFrom(objectType))
            return base.CreateContract(objectType.BaseType);
        else
            return base.CreateContract(objectType);
    }
}