EF Core ProjectablesFlexible projection magic for EF Core

// ❌ Repeated 4× in a single query — change the formula and hunt down every copy
var orders = dbContext.Orders
    .Where(o => o.Lines.Sum(l => l.Quantity * l.UnitPrice) > 500)
    .OrderByDescending(o => o.Lines.Sum(l => l.Quantity * l.UnitPrice) * (1 + o.TaxRate))
    .Select(o => new
    {
        Total = o.Lines.Sum(l => l.Quantity * l.UnitPrice) * (1 + o.TaxRate),
        Tier  = o.Lines.Sum(l => l.Quantity * l.UnitPrice) > 1000 ? "Premium" : "Standard"
    })
    .ToList();

// ✅ Business logic lives on the entity — queries stay clean
class Order
{
    public decimal TaxRate { get; set; }
    public ICollection<OrderLine> Lines { get; set; }

    [Projectable]
    public decimal Subtotal => Lines.Sum(l => l.Quantity * l.UnitPrice);

    [Projectable]
    public decimal Total => Subtotal * (1 + TaxRate);   // composes ↑

    [Projectable]
    public string Tier => Subtotal switch               // pattern matching → SQL CASE
    {
        > 1000 => "Premium",
        > 250  => "Standard",
        _      => "Basic"
    };
}

var orders = dbContext.Orders
    .Where(o => o.Subtotal > 500)          // → WHERE
    .OrderByDescending(o => o.Total)       // → ORDER BY
    .Select(o => new { o.Total, o.Tier })  // → SELECT
    .ToList();