Get property which is calculated in select query

[Borlay]

I have plain sql with paging and search syntax, i want to execute it map tables to object and take total count. My code is like that:

var selectSql = " SELECT TotalCount = COUNT(*) OVER(), E.* FROM " + fullName + " E " + whereSql;
var rows = context.Set<EventTable>()
              .FromSql<EventTable>(selectSql, parameters.Select(p => p.Value).ToArray())
             .ToArray();
var totalCount = context.Entry(rows.First()).Property<int>("TotalCount").CurrentValue;

But i have problem when i trying to take property that not exist in database table. I don't need this property in a table, i just want to calculate it in select query. Why i can't do this?
In older version i can have class like below and map query into this class, in new framework its does not work because its throwing exception that entity of this type not found or something like that (don't remember now). If i add this new class in context, then its throwing new exceptions about discriminator property and etc.

[NotMapped]
public class EventSearchTable : EventTable
{
    [Column("total_count")]
    [Required]
    public int TotalCount { get; set; }
}

[rowanmiller]

The ability to map to classes/properties that are not part of the model/database is tracked by #1862 and is not currently implemented in EF Core.