Linq to SQL, How to call a function on a column from the selected row in each single row selection

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C#

LINQ

I have a linq to sql application. Inside it I have a method getGlassLens() that selects some columns from db and for each returned row builds a new instance of a class Glass_Lens.

I need to call another functions for each selected row, this fuction getGlassLensCats() returns a value which is used in the constructor of the class "Glass_Lens". This function returns a value that is not directly contained in the database, so it performs some operations to specify the value to be returned.

my question is how to call this function for each selected row? I tried inside the query, but clearly, it has no translation into sql

public List <Glass_Lens> getGlassLens (long cat_id)
   {
       DataClasses1DataContext dc = new DataClasses1DataContext();
       var q =
       from a in dc.GetTable<glass_len>()
       //orderby a.id descending
       select a;

       if (cat_id != 0)
           q = q.Where(a => a.cat_id.Equals(cat_id));

       var result =
       q.Select(a => new Glass_Lens(a.id, a.piece_cost, a.piece_price, a.number,    a.insert_date, a.cat_id, getGlassLensCats(a.id).First().ToString())/* Here where I want to call my function but actually this makes no sense*/)
        .ToList();
       return result;
   }