How should save objects in the database which are no entities

Question

An entity "schoolyear" has configurable visible week days which are visible or not in a time planner.

I see now 2 posibilites to save the visible week days.

Either I make an N:M relation: N schoolyear has M VisibleWeekDays

Or I put the visible weeks days in a comma separated field: "0,2,5"

The numbers are the enum value from the DayOfWeek enum.

Should I really go this overhead way of create another 2 tables (junction + VisibleWeekDay table) just to hold in one field the index of a day?

How would you save that visible week days?

Answer 1

Comma separation in a DB field hurts my feelings. Consider a bitmask:

[Flags]
public enum DayOfWeek
{
   Undefined = 0,
   Mon = 1,
   Tue = 2,
   Wed = 4,
   Thu = 8,
   Fri = 16,
   Sat = 32,
   Sun = 64
}

DayOfWeek bitmask = DayOfWeek.Mon | DayOfWeek.Wed | DayOfWeek.Sat;

This way you maintain sanity and query-ability.

To query this in SQL:

DECLARE @bitmast INT;
SET @bitmask = 64 | 1 | 16; -- sun, mon, fri

SELECT * FROM tbl
WHERE DayOfWeekBitMask & @bitmask = @bitmask;

To query this with LINQ to Entities:

int bitmask = DayOfWeek.Sun | DayOfWeek.Mon |DayOfWeek.Fri;
var query = Context.tbl.Where(r => (r.DayOfWeekBitMask & bitmask) == bitmask);

To persist:

int bitmask = DayOfWeek.Sun | DayOfWeek.Mon |DayOfWeek.Fri;
var item = Context.tbl.First();

item.DayOfWeekBitMask = bitmask;
Context.SaveChanges();