LINQ and C #: how to add a field and not mapped to a table column

I am loading records from a database table. I need to add a field called "temp" that doesn't match any columns in the table. How do I add this field?

namespace mynamespace.Models
{
    public class alert
    {
        [Key]
        public int id { get; set; }
        public string text { get; set; }
        public string temp { get; set; } //<--- this generates error
    }
}

      

        
        
        
      

    

Thank you and welcome.