Repository pattern method implementation with entity framework and C#

Question

I have two entities in my database: Employee and Shift. I want to implement a C# repository pattern method, that given two workshift IDs, swaps the two workshifts for two employees.

Limitation: one employee cannot work multiple shifts at a time.

Models

public class Employee
{
    public int ID { get; set; }
    public string Firstname { get; set; }
    public string Lastname { get; set; }
    public string Email { get; set; }
}

public class WorkShift
{
    public int ID { get; set; }
    public int EmployeeID { get; set; }
    public DateTime ShiftStart { get; set; }
    public DateTime ShiftEnd { get; set; }
}

Repository

public class WorkShiftRepository : RepositoryBase<WorkShift>, IWorkShiftRepository
{
    private readonly ShiftContext _databaseContext;

    public WorkShiftRepository(ShiftContext context) : base(context)
    {
        _databaseContext = context;
    }

    private async Task<bool> Overlaps(WorkShift shift, int employeeId)
    {
        return await _databaseContext.Shifts.Where(u => u.EmployeeID == employeeId).
            Where((s => shift.ShiftStart < s.ShiftEnd && s.ShiftStart < shift.ShiftEnd)).AnyAsync();
    }

    public async Task<int> SwapShifts(int shiftIdA, int shiftIdB)
    {
        var result = 0;
        // Load shifts into memory
        var shiftA = await _databaseContext.Shifts.SingleAsync(s => s.ID == shiftIdA);
        var shiftB = await _databaseContext.Shifts.SingleAsync(s => s.ID == shiftIdB);
        int shiftAid = shiftA.EmployeeID;

        // Swap Employee IDs
        shiftA.EmployeeID = shiftB.EmployeeID;
        shiftB.EmployeeID = shiftAid;

        // Check if the swapped shifts has any overlaps on db
        if (await Overlaps(shiftA, shiftA.EmployeeID) || await Overlaps(shiftB, shiftB.EmployeeID))
            throw new Exception("ShiftsOverlap");

        // update in memory
        _databaseContext.Shifts.Update(shiftA);
        _databaseContext.Shifts.Update(shiftB);

        // Write changes to DB
        result = await _databaseContext.SaveChangesAsync();

        return result;
    }
}

The code compiles and it meets the limitations. But I am looking for tips and tricks to make the code cleaner and/or more efficient.