Question

I have an existing set of items in a database where a MultiSelectList of Id’s allows Shifts to be added to an Operator.
Currently, an Operator has 3 Shifts assigned. This is now edited on the View to remove 1 Shift from the 3.

What is the best way to go through the existing set of items and determine which of the 3 to remove that isn’t found in the list of selected items.

I know the easiest method would be to remove all items in the database for the Operator and then insert the values from the MultiSelectList but thought there might be a way to use LINQ to select from existing where not in the list of Id’s

var existing = (from os in db.OperatorShifts
                where os.OperatorId == model.Id
                select os);

public List<Guid?> Shifts { get; set; }
public MultiSelectList AvailableShifts { get; set; }

model.Shifts

Answer 1

//asuming your db has these fields: db.Shifts, db.OperatorShifts
//and model is the current Operator

var existingShifts = (from os in db.OperatorShifts
                where os.OperatorId == model.Id
                select os).ToList();

IEnumerable<Guid> newShiftIds = ?; //don't now how your selected shift ids got post back, figure it out yourself

var shiftsToRemove = existingShifts.Where(e => newShiftIds.All(id => e.Id != id)).ToList();
var shiftIdsToAppend = newShiftIds.Where(id => exising.All(e => e.Id != id)).ToList();

foreach(var shift in shiftsToRemove)
{
    db.OperatorShifts.Remove(shift);
}

foreach(var shiftId in shiftIdsToAppend)
{
    db.OperatorShifts.Add(new OperatorShift{
        OperatorId = model.Id,
        ShiftId = shiftId
    });
}